Anti-whaling group Sea Shepherd says its vessel 'Brigitte Bardot' has been struck by a rogue wave that has cracked the hull and is being pounded by heavy seas about 2400km southwest of Fremantle.
Sea Shepherd says the accident happened as the boat tailed Japanese whaling ship Nisshin Maru in six metre swells.
Captain Paul Watson, aboard the Sea Shepherd vessel Steve Irwin, said it was on its way to the Bridget Bardot to either escort the vessel or tow it back to Fremantle. The crew on board were reported safe.
"We're about 15 hours away," Captain Watson said.
"The captain has assured me that everybody's fine and they're not going to sink or anything."
Captain Watson said he was battling rough seas with five-to-six-metre waves.
"I believe a pontoon's probably broken, which means it'll probably have to be brought back to Fremantle for repairs, but it's certainly seaworthy," Captain Watson said.
"Most likely it'll have to be repaired.
"I don't know how long it'll take, but the Steve Irwin will then turn around and go back to join the Bob Barker in pursuit of the whaling fleet."
Meanwhile, the Sea Shepherd vessel Bob Barker will continue pursuing the Japanese whaling fleet.
Sea Shepherd claims to have saved more than 800 whales from being killed during last year's hunt, which Japan maintains is for scientific purposes, and says the industry is in such decline that this could be the final campaign.
Australia's Maritime Safety Authority (AMSA) said it had been monitoring the situation but there was no active rescue afoot because Sea Shepherd was managing the situation.
"We were aware of it, but it was really a monitoring brief for us because it was a Sea Shepherd vessel to which another Sea Shepherd vessel was going to the aid," an AMSA spokesman told AFP.
"We were in communication with them but they've got it under control themselves now."
Treasurer Wayne Swan said he was yet to be briefed on the incident.
"Naturally we are worried about the safety of anyone at sea, particularly in rough seas, and where the Sea Shepherd is at the moment," he told reporters in Brisbane.
Sea Shepherd purchased the Australian-flagged Brigitte Bardot, a high-speed 100-foot monohull racer, to replace the futuristic speedboat Ady Gil, which sank during a fierce clash with the harpooners in January 2010.
Watson said there were 10 crew on board the stricken Bardot -- three Britons, three Americans, an Australian, a Canadian, a Belgian and its South African captain Renecle.
EXCLUSIVE: On board 'Bob Irwin'
The crew of the Sea Shepherd's anti-whaling vessel MV Bob Barker show us around their ship and explain what it's like on a trip to the South Ocean to chase the Japanese whaling fleet.