But skipper Paul Watson and his crew were to face questions over the boarding of the Yushin Maru 2, which sparked heated Japanese accusations of "terrorism" and "piracy".

"Australian Federal Police will be making preliminary inquiries into the events that occurred in accordance with Australian legislation and Australian's obligations under international law," a police spokeswoman told Reuters.

The duo, Australian Benjamin Potts and Briton Giles Lane, were held for three days and whaling was suspended before they were handed to an Australian fisheries patrol vessel, which Japanese officials accused of giving them "limousine" service.

Potts, who plans to return to sea with Watson, said he had no regrets, despite his actions causing diplomatic friction between Australian and Japan.

"I think it's highlighted what's going on down there in the Antarctic," he said.

Police said an investigation had not officially started as officers were looking at whether international sea laws had been broken by either the protesters or the Japanese.

"If Australian wants to charge an Australian with piracy for trying to stop illegal whaling by the Japanese in the Antarctic territory, that's going to be a very amusing trial," Watson said.

The spokeswoman said an investigation could also be launched if Japan's government formally referred it to Australian authorities.

Japan has accused Australia of being too lenient on the pair because Prime Minister Kevin Rudd has promised a tougher approach, possibly including an international legal challenge, against Tokyo's cull of close to 1,000 whales.

Australia's Foreign Minister Stephen Smith met Japan's Prime Minister Yasuo Fukuda on Friday and they agreed not to let Canberra's opposition to whaling "influence diplomatic negotiations".

Japan has been Australia's biggest trading partner since 1957. The two countries signed a defense pact last year and are in talks toward a free trade agreement, though this has been slowed by Japan's reluctance to open up its agricultural sector.

Australia's Home Affairs Minister, Bob Debus, told local radio that at least five whales had been so far been taken by the Japanese whale boats since Greenpeace and Sea Shepherd protesters both left the Southern Ocean to refuel.

Japan plans to hunt almost 1,000 minke and fin whales for research over the Antarctic summer.

Despite a moratorium on whaling, Japan is allowed an annual "scientific" hunt, arguing whaling is a cherished tradition and the hunt is necessary to study whales. Its fleet has killed 7,000 Antarctic minkes over the past 20 years.

Sea Shepherd's Watson said he was not worried about police inquires and planned to return south to chase the six-ship Japanese whaling fleet as soon as the Steve Irwin completed re-supply and engine repairs in around 10 more days.

"By far the most effective method is to keep chasing the fleet. They cannot do anything while we are following them and getting in their way," he said.