Former Sea Shepherd activist Peter Bethune has accused the conservation group of scuttling one of its own ships as a publicity stunt after it collided with a Japanese whaler.

Mr Bethune labelled the Sea Shepherd's leadership "morally bankrupt" for allegedly ordering the hi-tech trimaran Ady Gil to be scuttled after it collided with a Japanese whaler in the Southern Ocean in January.

Mr Bethune was the Ady Gil's captain at the time and says Sea Shepherd founder Paul Watson ordered the ship be sunk to "garner sympathy with the public and to create better TV" in the publicity battle against Japan's Antarctic whaling program.

"It was definitely salvageable. It was still rock-solid from the engine room back," he said.

Mr Bethune spent five months in custody in Japan earlier this year after illegally boarding the same whaling ship a month after the high-seas collision.

He says he had cut all ties with Sea Shepherd.

"I think an organisation that relies on public money and public generosity to survive has an obligation to be honest," he said.

Mr Watson denies the allegations, saying Mr Bethune is bitter over his falling out with the organisation.

"No-one ordered him to scuttle it. Pete Bethune was captain of the Ady Gil. All decisions on the Ady Gil were his," he said.

Sea Shepherd distanced itself from Mr Bethune when he was awaiting trial in Japan for boarding the Shonan Maru II, but later said it was a ploy to try to ensure the activist received a light sentence.

Mr Bethune was given a two-year suspended sentence in the Tokyo District Court in July after pleading guilty to obstructing commercial activities, trespass, vandalism and carrying a knife, which he used to cut the ship's security netting.

Japan hunts whales in Antarctic waters using a loophole in a 1986 international moratorium that allows "lethal research".

The hunt has resulted in a spate of high-seas confrontations in recent years as conservation groups, such as Sea Shepherd and Greenpeace, seek to disrupt the Japanese fleet's activities.

- AFP