The Sea Shepherd group says banning Pete Bethune from its Antarctic anti-whaling missions was just a legal strategy during his trial, and the New Zealand activist is welcome to join them on future trips.

Bethune, 45, was handed a two-year suspended sentence in Tokyo yesterday after he was convicted on five charges relating to clashes with Japanese whalers in Antarctic waters in which he hurled rancid butter and scaled a Japanese vessel.

The Aucklander's family was told yesterday he will leave Japan tomorrow and return home by the weekend, after five months behind bars.

Bethune was aboard the futuristic trimaran Ady Gil when it sank after a January 6 collision with the Japanese whaling fleet's security ship the Shonan Maru II.

A month later, Bethune boarded the Japanese ship to demand compensation but was detained and taken to Japan for prosecution, where he has been in custody since February.

The Sea Shepherd group in June banned Bethune from taking part in further anti-whaling protests but the group's leader, Paul Watson, said the move was just a legal strategy during the trial.

"I don't think he'll be going back this season, because I think he's going to be writing a book, which is good, but he's certainly welcome back in the future," he told Radio New Zealand.

Mr Watson vowed to return to the southern seas in the coming whaling season to cause as much disruption to the expected Japanese hunt as possible, and said that was partially due to a lack of government action.

"We suspect it will be more aggressive. New Zealand and Australia have given Japan a green light to be more violent by not doing anything," he said.

"We are going to go down there stronger than ever and I have a couple of thousand crew applications, so we are not going to have trouble finding crew we are not going to retreat, these people are poachers, they are targeting endangered whales in the Southern Ocean whale sanctuary and it's illegal."

Japan was only getting away with it because of its power as a trading nation, Mr Watson said.

His comments go against urgings from Prime Minister John Key yesterday to keep "cool heads" while the international community struggles to come up with a compromise suitable to whaling and non-whaling countries.

"In the end we want to see an end to whaling in the Southern Ocean," he said. "We strongly believe the kind of actions we've seen will ultimately lead to a loss of life if we're not careful, so trying to find a solution which is acceptable is very important."

Mr Key said there would be plenty of relief from Bethune and his family over the suspended sentence, but was not prepared to pass judgment on the decision made by Japanese judicial authorities.