Wellington, July 9 NZPA - The outgoing Japanese ambassador in Canberra has urged Australia to take a more "robust" attitude toward the activities of the Sea Shepherd activists -- such as New Zealander Pete Bethune -- in the Southern Ocean.

Ambassador Takaaki Kojima told the ABC many Japanese people had been confused by Australia's failure to act against what he claimed were often violent acts against the whalers.

Japan's whaling fleet was conducting a legitimate activity under international law, and deserved protection, Mr Kojima said.

"With regard to Sea Shepherd...the Australian government should show a more robust attitude towards their violent activities," he said.

Bethune's futuristic trimaran Earthrace -- renamed the Ady Gil when it was bought by the US-based Sea Shepherd Conservation Society -- sailed from an Australian port on the protest voyage, as did the society's flagship vessel Steve Irwin.

"The Australian Government...[should] take appropriate measures against Sea Shepherd to punish and deter their violent activities," said Mr Kojima.

New Zealand Prime Minister John Key said in the wake of Bethune being sentenced in Tokyo District Court yesterday to two years' jail, suspended for five years, that there was a limit to how far anti-whaling protesters should go.

Mr Key has called for "cool heads" when it comes to anti-whaling protests in the Southern Ocean.

"We strongly believe that these kinds of actions will ultimately lead to a loss of life if we're not careful," he said.

The Sea Shepherd anti-whaling group banned Bethune from the group during his trial, but has since said that was a legal strategy and he was welcome to join future missions.

The judge in Tokyo said the court had suspended the jail term because Bethune had no criminal record in Japan, had apologised and paid for damage he caused, and because he had said he would join no more Antarctic missions.

But the group's leader, Paul Watson, told Radio New Zealand the pledge on future action was just a legal manoeuvre as Japanese judges would have been hesitant to suspend the sentence if he were to return to the Southern Ocean.

"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," said Watson, who has vowed to return to Antarctic waters in the next whaling season to disrupt the expected Japanese hunt.

Bethune has apologised to his family for the trouble and worry he has caused, the ABC reported.

He said he was desperate to get back home and see his family, including his two teenage daughters, Danielle, 15, and Alycia, 13.