Sea Shepherd leader Paul Watson told media following anti-whaling activist Peter Bethune's sentencing that because of a deal that had been negotiated with the Japanese, Mr Bethune would not be on the next Sea Shepherd protest.

"We can't have him come on the next Antarctic campaign because of a deal we made with the Japanese," Mr Watson told TV3 news. "But he'll certainly be welcome to be involved with us."

Mr Watson said the defence had cost the Sea Shepherd in excess of $500,000.

Peter Bethune was sentenced to a suspended prison sentence that will see him kept out of jail, provided he stays out of trouble with Japanese authorities for the next five years. Reports out of Japan suggest he will be deported immediately.

Mr Bethune's family has spoken of their relief as news filtered through that their son Peter has received a suspended sentence.

Don Bethune has not yet had a chance to talk to his son on the phone but said the message came from lawyers in Japan via text message.

"It is a terrific relief," Mr Bethune said.

Outside the court, about 30 right-wing protesters chanted and held up placards, including one that said, "Give Sea Shepherd terrorist capital punishment".

Shuhei Nishimura, one of the protesters, called the sentence
too lenient".

Don Bethune described the charges that his son faced as "bogus". Bethune had pleaded guilty to four charges including trespassing, vandalism and holding a knife, which he used to cut netting as he climbed on to the ship from a jet ski.

Mr Bethune said his son had carried a knife in the outdoors ever since his days in the Boys Brigade.

But Don Bethune said the arrest, detainment and the trial would never have gone ahead if the New Zealand Government had taken a hardline approach to Japan's whaling in the Southern Ocean.

Mr Bethune said the New Zealand Maritime Authority has yet to issue a finding on the causes of the collision between his son's vessel the Ady Gil and a Japanese whaling ship, the Shonan Maru 2.

"My firm belief is that if the New Zealand and Australian maritime authorities had worked together, I am quite convinced that they would not have put Pete in jail, he would have landed in Sydney and they would have called it quits," Mr Bethune said.

He said the only reason the New Zealand Government had not made a finding on the collision was through fear that it could damage trade between the two countries.

Pete Bethune climbed aboard the Shonan Maru 2 in February to confront its captain over the sinking of his protest vessel the previous month. The former Sea Shepherd activist was arrested when the boat returned to Japan in March.

The Sea Shepherd Conservation Group has been protesting against Japan's whaling which it carries out under a loophole in the international moratorium that allows killing of the ocean giants for "scientific research".

Speaking to Radio New Zealand, Bethune's wife Sharyn said it was a relief to hear her husband will no longer be held in custody in Japan.

"I'm just standing here taking it in. I'm about to open a bottle of bubbles to celebrate," she said.

Mrs Bethune said she was not sure exactly how long it would be before Peter was back in New Zealand, but said he should "hurry up and get back here before [the Japanese] change their mind".

Mrs Bethune told Newstalk ZB she initially thought her husband would spend two years in prison, but was elated when she realised he would soon be back in New Zealand:

"When we clarified that [the sentence] was actually suspended, it was like 'oh my God, what a relief'. It's just amazing. I think they realised he's not a bad person. It's just a good outcome all around," she said.

"I think it was everyone's support and love that got it through and got us a positive outcome."

Mrs Bethune said the announcement ended what had been a long ordeal.

"For me it wasn't so bad, my life went on. But for Pete, spending 23 hours a day in a cell, the time must have dragged."

She said she was still trying to contact one of her daughters to tell her the good news.

Mrs Bethune said flight details were not yet confirmed, but "we're expecting him home on Saturday".

"But someone did say that might get him out straight away, and in that case he could be home Thursday," she added.

Prime Minister John Key has stopped short of commenting on the sentence handed down to anti-whaling activist Peter Bethune but says it would have come as a huge relief to him and his family.

Mr Key said, as was the case with New Zealand judicial decisions, the Government would not pass judgment on today's sentence.

"But at the end of the day there is now a situation where Pete Bethune is returning home. For him he doesn't have to spend time in a Japanese prison and whatever his motivation is for the particular incident, he is going to be relieved...''

Maritime New Zealand is investigating the incident but there is no indication of when an outcome might be reached.

Mr Key said the whaling situation had been tense and that was why New Zealand preferred pursuing a diplomatic solution.

Those attempts stalled indefinitely after an International Whaling Commission meeting failed to get a compromise last month.

Mr Key said it would be at least a year until such talks resumed and "cool heads" needed to prevail in the meantime as there was a real risk of death if clashes in the Southern Ocean continued.

Labour's conservation spokesman Chris Carter also welcomed the news that Bethune would return home.

"I am a long-standing critic of Japan's whaling policy," he said. "However, I applaud the Japanese justice system for its humane approach, which has recognised that Peter pleaded guilty to the charges he was facing in Tokyo but has not acted like a terrorist or a hijacker."

The Green Party hailed Bethune as a hero.

Green Party co-leader Russel Norman said thanks to Bethune, many whales had been saved.

He criticised the Government's "lethargic attitude" to whaling.

"Not only has the New Zealand Government spent years not enforcing the Moratorium on Commercial Whaling and the Southern Ocean Whale Sanctuary, but it has left the Kiwi activists who do so high and dry by not pursuing the Maritime New Zealand Investigation," Dr Norman said.

"If New Zealand stood by its traditional anti-whaling allies and took a case to the International Court of Justice for an immediate injunction on whaling in the Southern Ocean Sanctuary, we could have a result as early as next year. With no whaling in the sanctuary, there will be no whale wars with activists," he said.

- Edward Gay, NZ Herald Staff & AP