An emotional Pete Bethune has returned to New Zealand unsure if he will head down to the Antarctic again but vowing he will never give up the fight to stop Japanese whaling.

The Tokyo District Court on Wednesday sentenced Bethune, 45, to two years' jail, suspended for five years, for assaulting a whaler by hurling a rancid butter stink bomb during a high-seas confrontation.

He pleaded guilty to obstructing commercial activities and charges stemming from climbing aboard the Japanese ship - trespass, vandalism and carrying a knife, with which he cut the ship's security netting.

But he denied the assault, which prosecutors said left a 24-year-old whaler with chemical splash burns to his face following the February 11 confrontation in which Sea Shepherd activists hurled the stink bombs.

Mr Bethune, who spent four months in a Tokyo jail, told a media throng after touching down in Auckland today that he wanted a coffee.

"I want to go to a coffee shop. In prison, there's nothing to do. You are stuck in a cell 23-1/2 hours a day and I have been looking forward to going to a coffee shop and having a coffee and a muffin."

He said he had no regrets about the experience.

"Whatever you do in life, there will always be people who don't support you. But I stand by what I did."

He told TV3 he had "no complaints" about his treatment at the hands of Japanese authorities.

"My complaint is just that they have been so poor in investigating the criminal acts of the whalers," he said.

"I was more worried about the physical side in prison.

"I think I was more worried about being beaten up and shagged by sumo wrestlers and stuff - there was none of that."

He left the airport with his wife, Sharyn, and daughters Danielle, 15, and Alycia, 13.

The Aucklander became an anti-whaling activist in the United States-based Sea Shepherd Conservation Society when he sold his record-setting carbon-fibre and kevlar trimaran Earthrace to the society. Ady Gil, the Hollywood businessman who put up the money, wanted the boat re-named after him, and for Bethune to keep piloting it.

The futuristic-looking boat sank after a January 6 collision with the Japanese whaling fleet's security ship the Shonan Maru II, and a month later Bethune boarded the Japanese ship to demand compensation.

He was detained and tried in Tokyo.