The Sea Shepherd anti-whaling group says it has intercepted a Japanese whaling supply ship in the Southern Ocean and plans to stop it delivering its cargo to the fleet's factory ship and harpoon vessels.

The Sea Shepherd ship, Gojira said it was tailing the tanker, the Panamanian registered Sun Laurel carrying fuel and other supplies.

When the Gojira captain Canadian Lockhart MacLean found the San Laurel he immediately alerted other Sea Shepherd ships in the area which changed course to intercept the tanker.

Captain Paul Watson said from the Sea Shepherd ship Steve Irwin described the San Laurel as the "Achilles heel" of the whaling fleet.

"We intend to stay on it like a bloodhound to keep this ship from delivering fuel and supplies to the whaling fleet.

"This tanker's support of Japan's illegal activities makes the captain and crew of the Sun Laurel as culpable as the person firing the harpoon into a whale's flesh."

He said the Japanese whaling ships would need to refuel soon.

"We will be here with the Sun Laurel waiting for them to arrive. Refuelling south of 60 degrees is illegal and we intend to enforce the Antarctic Treaty if they attempt to violate it."

He said there was no evidence any whales had been killed this season as the whaling fleet had spent the past two weeks fleeing from the Sea Shepherd ships.