Australia on Thursday offered to send a ship to secure the freedom of two anti-whaling activists held by Japanese whalers in Antarctica.

Australian Benjamin Potts, 28, and Briton Giles Lane, 35, have been detained since they forced their way aboard a Japanese harpoon vessel on Tuesday.

A stalemate has developed with the Japanese refusing to release their hostages until the Sea Shepherd activists aboard the Steve Irwin agree to call off their protest.

Foreign Affairs Minister Stephen Smith said the Oceanic Viking, a Customs ship sent by Canberra to monitor the Japanese whaling fleet, was in the vicinity and ready to take off the two protestors.

"We now need the full and complete cooperation of the two vessels, the two captains and the two men concerned," Smith said.

Unless a deal is brokered for the transfer to the Oceanic Viking before the pair are returned to the Steve Irwin, Potts and Lane could stay in custody until the whalers reach Japan.

They could then be put on trial for piracy because they boarded a vessel without permission.

Paul Watson, captain of the Steve Irwin, was adamant about not accepting any conditions for the release. He also warned of a possible commando-style raid to free the pair by force.

"I'm not going to allow them to take them back to Japan and put them on trial for piracy," Watson said. "We have an obligation to get our people back, one way or another."

In Tokyo, meanwhile, the Japanese government Thursday offered a different slant on the standoff. It accused activists aboard the Sea Shepherd of a lack of responsibility for not picking up their two colleagues on the Japanese whaling ship.

"It is a truly bizarre situation that they would not come to pick (the members) up," Chief Cabinet Secretary Nobutaka Machimura said at a press conference.

The Steve Irwin was reported around 70 kilometres behind the Japanese ship holding the pair. The ships were more than 4,000 kilometres south-west of Perth on Australia's west coast.