Wellington, March 16 NZPA - The Government says it hopes the Japanese government will release anti-whaler Peter Bethune but will not interfere in Japan's court processes.

The Green Party today demanded the Government use its diplomatic clout to push for the release of Mr Bethune, 44, who was arrested on Friday and remains in custody in Japan.

He was captain of the protest organisation Sea Shepherd's high-tech powerboat the Ady Gil that was sliced in two and sank a day after a collision on January 6 with the whaling ship the Shonan Maru II. In mid-February Mr Bethune climbed aboard the Japanese ship intending to make a citizen's arrest of captain Hiroyuki Komiya for what he said was the attempted murder of his six crew. He also presented a $US3 million ($NZ4.3 million) bill for the Ady Gil, a carbon-and-kevlar trimaran.

Green Party co-leader Metiria Turei said Mr Bethune boarded the boat when it was in ocean managed by Australia and there were questions about what jurisdiction his case should be heard under.

"I'm not convinced that he has committed an unlawful act... He boarded the vessel with a lawful purpose and I think that that legal uncertainty along with the jurisdictional uncertainty means that New Zealand should take a stronger position in protecting a New Zealand citizen from the Japanese law," she told reporters.

She said the citizen's arrest attempt was allowed under maritime law.

In Parliament Green MP Gareth Hughes asked Foreign Minister Murray McCully to request the Japanese government to release Mr Bethune.

Mr McCully said Mr Bethune was receiving consular support.

"When Mr Bethune stepped aboard the Shonan Maru he rendered himself liable to the Japanese legal jurisdiction as a consequence of the United Nations convention of the law of the sea. That convention recognises that vessels on the high seas are under exclusive jurisdiction of the flag state of the vessel," Mr McCully said.

"The New Zealand Government naturally hopes the Japanese authorities will release Mr Bethune. However we accept that he has been charged with trespass and this matter needs to be resolved by the Japanese judicial system... Just as New Zealand's judicial system operates independently of all outside directions we cannot seek to interfere in the judicial processes of another country."

Mr McCully said he talked to the Japanese Ambassador in New Zealand Toshihiro Takahashi and had phoned Japan's Foreign Minister Katsuya Okada about Mr Bethune.

Sea Shepherd, which has called Mr Bethune the first New Zealander taken as a "prisoner of war" to Japan since World War 2, is working on his legal defence.

The group declared an end to this season's three-month pursuit of Japanese harpoon ships in Antarctic waters on February 27, saying it had been the most successful campaign yet because it had stopped all whaling on 33 days.

Mr Hughes told reporters that whaling should be on trial not Mr Bethune and the activist was doing more than the Government to oppose the activity.

"We have to ask some questions about what sort of a trial he is going to face in a Japan which is so blatantly pro-whaling and has been for a number of years.

"Let's not just sit on the sidelines, let's get a bit more involved in this case."

The New Zealand Government says it will consider backing Australia should it decide to take a case to the International Court of Justice if a diplomatic solution is not reached.

Hundreds of whales are caught every year under a scientific research loophole and negotiators are studying a proposal ahead of a June International Whaling Commission meeting in Morocco that would let Japan, Norway and Iceland hunt whales openly despite the moratorium but reduce the catch "significantly" over 10 years. Australia has rejected the proposal.