The Sydneysiders couldn't have done it alone. But through a brother fighting with Islamic State in the Middle East, they plotted with a senior foreign "controller" who provided the weapons and the instructions, police have said.

It is a reminder that even as the so-called caliphate is broken up across Syria and Iraq, two years of acting as a quasi-nation has given IS capabilities, experience, ambition and influence that will roll on - in one form or another - for years.

One plot of such ambition and technical capability does not make a trend. Low-sophistication, spontaneous attacks will remain the most likely kind of thing we'll see, police and experts say.

But the allegations do align with two messages the security agencies have been sending for some time: complex attacks on hard targets are still very much in the jihadist playbook, and the destruction of the so-called caliphate - which ought to take some of the gloss off Islamic State triumphalism - won't diminish the Islamist threat in Australia.

The latter point is especially depressing. There were humanitarian and international security arguments for joining the military campaign against IS, but part of the reasoning was that smashing IS's heartland would make Australia safer.

Perhaps it will in the long run, but expert prognostications are that as IS disperses and spawns new groups that evolve their own agendas and methods, it will be a long time before the benefits flow through. Fairfax Media has spoken to a range of past and present security officials. The majority view is that while we are entering a new phase, it will be no less dangerous.

"It's not a comforting thought but I think a heightened level of alert is essentially now the new normal," Justice Minister Michael Keenan said.

"I can't see a time in the medium term where that's going to revert back to what it was pre-2014."

Keenan said the "the ideology will be around" for years but would evolve, probably into "offshoots of those existing organisations".

Radical Islamism has reinvented itself many times before. There was an al-Qaeda era in which hatred of the United States drove terror attacks that changed the world and spawned fellow travellers that attacked other countries involved in Middle East conflicts.

Then there was an Islamic State era in which the remnants of al-Qaeda splinter groups - too extreme ironically for the parent organisation that had murdered thousands of people - seeded a group that opportunistically filled the vacuum left by the Syrian civil war and then did a smash-and-grab on a weak, sectarian Iraq.

It focussed its efforts on building its caliphate but, after the US-led coalition began to beat it back, launched punitive attacks on the West.

Now, Mosul in Iraq has been declared retaken from IS. Raqqa in Syria is encircled by an alliance of US-backed Kurdish and Arab fighters who will defeat IS sooner or later. The US has outlined a policy of "annihilation" which Defence Secretary James Mattis has openly said aims to kill foreign fighters in situ rather than have them escape to other parts of the world such as Europe or Australia to wreak vengeance there.

But the past weeks' events dramatically underscore the fact that the return of foreign fighters, as much as they have dominated the public list of concerns, won't be the main game for Australia - though Europe with its porous borders may be a different story.

Many remaining Australians will be killed and those who make it out will more likely go to places such as Libya, northern Nigeria and Yemen, where new caliphate offshoots may take root.

As one respected Muslim youth worker who works on the coalface said: "Anyone who has gone over there and is still there has sacrificed their lives for the cause. There's no reason for them to come back here. I can't see it happening."

Rather, the post-caliphate movement's most potent reach will be in skills and expertise - which can be shared electronically, most troublingly via encrypted communications that are difficult for authorities to tap.

Any reasonably complex plot requires technical ability and experience - more so than might be expected - which is why they tend to have overseas input, as was the case in the more elaborate previous plots such as Melbourne's 2016 Christmas Day and 2015 Anzac Day plots.

There is little substitute for conflict experience, said Jacinta Carroll a former long-serving national security official now with the Australian Strategic Policy Institute.

"It takes technical support and financial support and planning support. And it takes a bit of co-ordination and networks to bring together people who have those things."

Whatever form IS takes next, it will include battle-trained, experienced and desensitised jihadists who are well-networked, Carroll said.

Keenan said that "certainly within ISIL there is work being done on the idea of encouraging attacks on the western world and that becomes particularly acute the weaker they get because this will serve to show they're still in the game when we're rolling them back in their heartland".

The trans-national nature of the modern threat is also underscored by the fact that the alleged plot revealed this week was picked up by US and British intelligence partners.

In some ways, IS has been damaged by gradually losing its heartland that stretched across Syria and Iraq. Carroll said the assault on IS in Syria has all but destroyed its super-slick propaganda machine that made stars of Australians such as Neil Prakash and Abdullah Elmir.

Much of its original lustre came from the fact that it projected success, even invincibility, in creating its perverse utopia. Whether or not its military losses will dim enthusiasm among impressionable or angry young Muslims in the West remains uncertain.

The critical mass of online propaganda was at such a stage that even without the IS heartland, its message would continue to sustain itself, Keenan said.

"I believe it will still be quite easy to find this ideology online. We take it down at an enormous rate and the companies are getting better and better at identifying it. But it's an enormous challenge to eradicate it completely online."

Asked whether the trend towards younger people becoming radicalised quickly was tailing off, Keenan replied: "Despite our significant efforts on countering violent extremism, we're not at a point yet where we're seeing a lessening of young people being radicalised, and this is the case around the world."

That said, some insiders privately say they believe there will be a decline in enthusiasm without the drawcard of the caliphate. People who work with at-risk Muslim youths also say they're seeing it. One who helps steer troubled young people away from extremism but cannot be identified because of the sensitive nature of his work said he believed the baseline of enthusiasm had shrunk.

"The ones who are going to get sucked in now are the fringe of the fringe of the fringe. People who were ISIS supporters are not any more," he said.

Andrew Zammit, terrorism researcher who is highly regarded for the rigour of his analyses, said IS' loss of territory wouldn't necessarily stop them from inspiring attacks from abroad or guiding them remotely.

The group has lost some key people who had guided plots from abroad including high-profile English-speaking jihadists such as British former hacker Junaid Hussain, who helped direct the Melbourne Mother's Day plot, as well as others in Britain and the US.

"We'll have to see how they adapt," said Zammit to losing such remote instructors. But he pointed out that some were based elsewhere, in Libya and Somalia.

There were also plenty of other groups that could create a threat to Australia, Zammit said, such as the al-Qaeda aligned Hayat Tahrir al-Sham, which controls a lot of territory in Syria, and al-Qaeda's branches in Yemen and Libya as well as a resurgent al-Shabaab in Somalia.

On the whole, Zammit said IS military defeats in Syria and Iraq "can't be assumed to reduce the threat level here".

The other major question is how Islamic State disperses. Keenan said the establishment of the caliphate was "a shot of adrenaline to every radical Islamist around the world" and while it was important to destroy its base, "there's a good chance that in some shape or form they will re-establish themselves ... in other parts of the world."

The most pressing worry for Australia is the southern Philippines. Amid the flurry of attention to last weekend's raids, little notice was given to an important visit by Attorney-General George Brandis to Indonesia to improve co-ordination in south-east Asia on the movement of jihadists and weapons across borders.

While Australia's co-operation with Indonesia is strong, sources say much more work has to be done between countries in the region, notably Malaysia and the Philippines.

"This is important for Australia because a lot of this supported effort is going into the Philippines because of weak borders in the tri-border area," said ASPI's Carroll.

Not only have southern Philippines groups pledged allegiance to IS, but IS leader Abu Bakr al-Baghdadi has returned the nod, giving the groups a significant brand and imprimatur. In the fight for the city of Marawi, locals are using tactics "straight from the IS playbook", Carroll said, and IS money is pouring into the area.

The possibility a mini-caliphate could take advantage of the poor border control and move people and weapons into Indonesia is a major concern given the number of Australians who travel and holiday in Indonesia, she said.

"That's a safe haven in our own backyard," she said.

Keenan said authorities hadn't seen Australians drawn to the Philippines as they had been to Syria but "the potential for them to entrench themselves there and then be a magnet for people either fleeing the Middle East or just within our own region to go to instead of the Middle East is a threat we are very, very alive to."

To keep attracting recruits and supporters in Australia, IS and its spawn will need a new narrative for the post-caliphate era. Several officials and experts flagged the possibility that the new jihadis could turn the defeat of the caliphate into a narrative of resentment.

"IS will still have its appeal, and can promote a narrative that it had created a glorious pure Islamic state that was destroyed by evil enemies," Zammit said.

Al-Qaeda was re-emphasising its anti-American focus, he said, most recently through speeches by Hamza bin Laden, the son of Osama.

The experienced Muslim youth worker said there were resentments in the community at how Australia has responded to the IS phenomenon - some though not all of which he sympathised with. New terrorism laws and the debate that has accompanied them had created a sense that Muslims were being targeted and marginalised, he said.

With frustrating circularity, many radicalised youngsters said they were angry at the fact that Australian troops are in the Middle East, he added.

"The number one reason [for becoming radicalised] they tell me is that we have troops over there," the youth worker said. "They see the whole of the government as the same instrument. The Army, state government, federal government. I'm not saying I agree with them but that's what they say."

Islamists have gone through many metamorphoses since the 1920s, when the Muslim Brotherhood was founded, one seasoned national security insider noted.

"Don't underestimate the capacity of extreme Salafists to again shape-shift and find some new narrative that speaks to a new generation of Muslims," he said.

"It's been commandeered by different people at different times and it'll happen again."