The Sea Shepherd ship Steve Irwin has collided with a harpoon boat in the Antarctic as the Japanese fleet shrugged off the hardline activists' attempts to halt whaling.

Sea Shepherd leader Paul Watson said his vessel hit the Yushin Maru No.1 when it swept in front of him to clear the Steve Irwin away from the stern of the factory ship Nisshin Maru so it could take a whale aboard.

Captain Watson said he tried to back away but the movement of the whaling ships made a collision inevitable. No one was hurt and the vessels were not seriously damaged.

He said the Steve Irwin recovered to be steaming on the stern of the Nisshin Maru and blocking further attempts at transfers.

So far today the whalers have managed to transfer four freshly harpooned minke whales to the Nisshin Maru, in the first successful whaling mounted by the fleet in the presence of Sea Shepherd.