Documents filed in the Federal Court allege a four-year-old boy, born in Darwin and detained on Christmas Island with his asylum seeker parents, was detained for no valid purpose and thus unlawfully.

Law firm Maurice Blackburn estimates "thousands" of asylum seekers are eligible to join the lawsuit, arguing their detention was unlawful because the government did not actively pursue their visa claims.

Mr Dutton would not comment on a matter before the courts, but has previously noted his department is the most litigated of all Commonwealth bureaucracies "due to Labor's failed border policies".

Only last month the Turnbull government settled a $70 million payout to nearly 2000 asylum seekers detained on Manus Island, plus $20 million in legal costs - avoiding a lengthy and difficult trial.

That was not an admission of liability, but "decisions on how to resolve legal matters are made on a case-by-case basis informed by legal advice", Mr Dutton said at the time.

Numerous challenges to the legality of Australia's immigration detention regime have failed, but Maurice Blackburn lawyer Jennifer Kanis said this lawsuit was based on new, untested claims.

"The points that we raise in our case have never been tested by the court," she said. "This case has potentially wide-ranging implications for the way Australia detains people who are seeking asylum."

The four-year-old lead plaintiff in the claim was detained for 480 days from September 2013. Lawyers allege the government failed to pursue his visa claim, nor was he detained for the purpose of removal offshore, and therefore his detention had no legal basis.

Once officials "turned their minds" to his claim, the boy was granted a bridging visa within three days, documents claim. The suit does not contend all detention is unlawful but that it "must only ever be used when there is a clear purpose", Ms Kanis said.

The claim was filed on October 19 and lawyers will next be heard in December.

Meanwhile, a scathing new report has substantiated fears about a looming tide of violence and unrest accompanying the closure of Australia's detention centre on Manus Island.

Human Rights Watch interviewed 40 refugees and asylum seekers, as well as service providers, in September, alleging a spate of brutal attacks that were never properly investigated.

One Iranian refugee said he was robbed at knifepoint by seemingly drunk young men armed with machetes, while another man was allegedly hit with a metal rod and sustained injuries so severe he had to be treated in Brisbane.

There were three serious attacks since June requiring medical evacuation, the group found, and many refugees now afraid leaving the regional processing centre.

Human Rights Watch Australia director Elaine Pearson said refugees' concerns about the centre's looming closure - and their decision to physically resist it - were justified.

"I don't blame them," she said. "The situation has deteriorated. Things feel very tense now on the island in a way that they didn't two years ago. It's a very volatile situation and I'm really quite concerned."

Ms Pearson said Australia should send Australian Federal Police officers to assist local Manusian police and "help ensure that crimes committed are fully investigated".

The Australian government has denied refugees are unsafe in Papua New Guinea. This week, Immigration Department bureaucrats told a Senate estimates hearing many men voluntarily made trips into Lorengau township every day.

Gillian Triggs, former boss of the Australian Human Rights Commission, also blasted the "illegal, cruel and unworkable" Manus regime in a fiery speech to refugee activists on Wednesday night.

She said Australia had a "shameful history of chronic failure to comply with our human rights obligations" and labelled it "ironic" Australia was now taking a seat on the UN Human Rights Council.