The conservation group Sea Shepherd says its ship the Bob Barker has been intentionally rammed by a Japanese whaling vessel off Antarctica.

Sea Shepherd says four Japanese harpoon ships were circling the Bob Barker because it was actively blocking the slipway of the Nisshin Maru - the Japanese whaling fleet's factory ship.

Sea Shepherd founder Paul Watson says the Yushin Maru 3 then intentionally rammed the Bob Barker, creating a hole in the side of the vessel above the waterline.

"The four harpoon vessels then began to circle the Bob Barker, being very intimidating," he said.

"At noon today one of the vessels, the Yushin Maru 3, came up close alongside and turned swiftly and then slammed its stern right into the stern end of the Bob Barker, cutting a 3-foot long, by four-inch wide gash in the hull."

The Sea Shepherd group said in a statement that no-one had been injured in the crash, which happened about 300 kilometres off Cape Darnley in the Australian Antarctic Territory.

Welding crews are working to repair the damage, which the group says is a deep gash to the starboard side of the vessel.

But Japanese whalers say they tried to avoid the collision.

In a statement, the Institute of Cetacean Research says the Bob Barker had become dangerously close to the Nisshin Maru, and were attacking it with lasers and bottles of butyric acid.

It says the Yushin Maru 3 moved to intervene, but was rammed by the anti-whaling ship.

On its website, the Sea Shepherd says the Bob Barker continues to block the Nisshin Maru in an effort to prevent the transfer of slaughtered whales.

Flagship vessel the Steve Irwin is still two days behind the Bob Barker, but Mr Watson is confident that when they arrive they will be able to halt whaling for the season.

"The damage is being repaired right now with our welding crew, but what it indicates is that the Japanese are being extremely aggressive this year," he said.

"It's not just whalers we're dealing with, we're dealing with Japanese security forces [who are] very disciplined, very aggressive."

It is the second major clash between the conservationists and Japanese whalers this year after the Ady Gil sank following a collision with a Japanese whaling ship in the Southern Ocean on January 6.

Opposition environment spokesman Greg Hunt says the Government needs to send an observation vessel to the Southern Ocean immediately.

"It is time now finally to send a vessel to observe what is going on, to be on hand in case of emergency and to capture and chronicle for the world the slaughter of whales in Australian waters by Japanese whalers," he said.

Japanese officials have not yet been available to comment.

-ABC/AAP