Anti-whaling group Sea Shepherd says a tracking device placed on a Japanese whaling vessel shows it is apparently returning to whaling grounds.

It was thought the Japanese fleet had finished its whaling season and was heading home.

But Sea Shepherd founder Paul Watson says a tracker on the Japanese tanker Sun Laurel shows it has made a sudden change of course and appears to be returning south.

Sea Shepherd vessel Bob Barker is in pursuit and returning to the Southern Ocean.

Mr Watson says he believes the whaling ship Nisshin Maru is heading back too.

"I don't know why they're doing this, it doesn't make any economic sense," he said.

"They won't be able to take many whales, but we don't want them to take any.

"It's a long ways back to the Southern Ocean, [they're] running out of time, they've only got about 10 days left until they can no longer stay down there."

Four Sea Shepherd ships have been following and documenting Japan's whaling vessels this season.

The hunt came to a head when Sea Shepherd boats collided with Japanese vessels during a re-fuelling operation.

Japan claims its whaling ship and the fuel tanker had to abandon the process after allegedly being struck five times by the Steve Irwin, Bob Barker and Sam Simon.

Sea Shepherd denies they rammed the vessels, but instead say they collided while trying to stay between the whaling ship and fuel tanker.

Yesterday Mr Watson said it had been a successful season for environmentalists, with the Japanese fleet taking its lowest number of whales in its history of hunting Antarctic waters.

Sea Shepherd claims the fleet killed no more than 75 whales.