When they steered their tinnies off Newport reef that hot, clear morning, the seven friends were not looking for a Japanese midget submarine. But five kilometres east of Bungan Head and 54 metres below the surface, Tony Hay spotted a propeller sticking out of the sea bed.

"I knew exactly what it was," Mr Hay, 55, said.

The existence of the midget submarine was no secret; M24 had become the naval historian's equivalent of the Loch Ness monster. Since its disappearance in the early hours of June 1, 1942, after its crew launched a daring night raid on Sydney Harbour, killing 21 Allied seamen with a torpedo, the submarine became the subject of endless speculation.

On Monday, the sixth anniversary of their discovery, the federal government granted special approval for the No Frills Divers to visit the wreck, which is now protected by a $1.1 million disturbance fine and an exclusion zone monitored by a long-range camera.

Nursing schooners at a Dee Why pub after their morning dive, the No Frills Divers said they were still curious about whether the two Japanese soldiers were inside the submarine. Their only clue is an escape ladder, which was never lowered, suggesting the men are still inside.

''But we'll never know,'' said Mr Hay.

His conjecture was confirmed by Tim Smith from the Office of Environment and Heritage, who had joined the men for beers.

Mr Smith said the submarine would be dangerous to recover due to the belief that it contains undetonated explosives, and besides that, the operation would be prohibitively expensive.

The No Frills Divers hope to make an annual visit to the wreck for as long as they are physically able to dive.

''This sort of thing doesn't happen in your lifetime as a diver,'' said Alan Simon, 65.