Australia is in ''diplomatic discussions'' over the fate of three activists who boarded a support ship for Japan's whaling fleet.
The activists from the Forest Rescue Australia group boarded the Shonan Maru No 2 early on Sunday, about 26km (16 miles) off the west coast of Australia.
The men could face charges under Japanese law, Attorney-General Nicola Roxon said.
This is not the first time activists have boarded Japanese whaling ships.
In 2010 New Zealander Pete Bethune boarded a ship which subsequently took him to Japan. He spent five months in jail after receiving a two-year suspended sentence and was later deported.
Forest Rescue said in a statement that the trio boarded the vessel to prevent it tailing a ship belonging to anti-whaling group Sea Shepherd.
That ship, the Steve Irwin, was on its way to the Southern Ocean, where Japanese whaling ships hunt at this time of the year, after escorting a damaged vessel back to Australia.
In the past there have been collisions between Sea Shepherd vessels and the whaling fleet. Last year the Japanese fleet returned home early, citing safety concerns, after confrontations.
There has been a ban on commercial whaling for 25 years, but Japan catches about 1,000 whales each year in what it says is a scientific research programme.
Critics say it is commercial whaling in another guise.
Trio 'being questioned'
The men, all from Western Australia, were named as Geoffrey Owen Tuxworth, 47, Simon Peterffy, 44, and Glen Pendlebury, 27.
The environmental group has demanded that their members be returned to Australia.
However, this could be ''tricky'' as the incident did not take place in Australian territorial waters, Ms Roxon said.
''Because it was only in our exclusive economic zone that doesn't give us automatic rights to assert Australian law,'' she told reporters.
''In fact, the clearest advice that we have is that Japanese law would be likely to apply,'' she added.
A spokesman for the Tokyo-based Institute of Cetacean Research told AP that the activists were not injured and were being questioned.
"I would describe them as volunteer detainees," said Glenn Inwood from the institute which sponsors Japan's annual whaling activities.
Last month, the Steve Irwin had to escort another Sea Shepherd vessel, the Brigitte Bardot, back to Fremantle for repairs after the latter was damaged by a rogue wave while chasing the Japanese fleet.
The Shonan Maru was on the tail of the Steve Irwin as it travelled back to Australia. It is thought to be watching the anti-whaling vessels to help the Japanese fleet avoid protesters.
But the presence of the Shonan Maru No 2 near Australian waters has not been well received.
"We've made very clear that this boat is not a welcome boat in our Exclusive Economic Zone," Ms Roxon said on Australian television.