An anti-whaling ship from the Sea Shepherd organisation has made a distress call after being damaged by a wave in the Southern Ocean off Western Australia.

The Brigitte Bardot was chasing a Japanese whaling ship in six-metre swells when a wave slammed into the port side of the vessel, cracking its hull.

The ship, which has a crew of 10, is 1,500 nautical miles south-west of Fremantle.

There are British, American, Australian, Canadian, Belgian and South Africans on board.

Skipper John Renecle is confident they will stay afloat until help arrives despite the crack getting wider as seas continue to pound his ship.

He says the vessel is taking on water which they are having to pump out.

"Certainly it's not an ideal situation; we're sitting in one of the roughest oceans of the world with a pontoon that's barely being held in place by our jury rig system," he said.

Paul Watson, who is on board Sea Shepherd flagship the Steve Irwin, says he is fighting heavy seas to reach the distressed ship.

He expects to reach it some time this evening.

"I mean these are six-metre swells and so if it comes down on the right angle, it's a freak sort of accident, but these things happen," he said.

"The captain reports that everybody's fine and they feel quite confident that they won't have any further problems, so we're on the way there."

The ship is taking on water but the crew is pumping it out and it is still afloat. ( Audience submitted: Sea Shepherd )

Risks

Mr Watson says the crews know the risk heading into the Southern Ocean.

"You are facing two dangers down here; it's not just the Japanese whaling fleet, it's also the weather conditions, the ice conditions and everything," he said.

"We've not had any problems before, but we've had eight campaigns down here, so it's inevitable at some point that something was going to happen."

He says he has some concerns because they are being tailed by a Japanese whaling boat.

"We're just worried they might be aggressive with a disabled vessel so we'll see what their intention are when we get there," he said.

Mr Watson says another anti-whaling ship, the Bob Barker, is still pursuing Japanese ship the Nisshin Maru.

"Last year the Bob Barker was able to stop the Nisshin Maru by itself so hopefully they will be able to do the same again," he said.

It is expected to take about five days to escort the distressed ship back to Fremantle.

Mr Watson says once the rescue mission is over, the Steve Irwin will rejoin the anti-whaling campaign in the Southern Ocean.