Japanese prosecutors yesterday demanded two years in prison for New Zealand anti-whaling activist Peter Bethune on trial for assault and charges relating to his boarding of a harpoon ship in Antarctic waters.

Bethune, 45, of Auckland, is accused of causing chemical burns to the face of a whaler with a rancid butter stink bomb and four other charges.

Chemical burns to the face of a 24-year-old whaler were "clearly caused by rancid butter fired by the defendant", prosecutors said.

"The impact of rancid butter and its toxic nature is clear from past studies, and our investigations found that the ship's floor was discoloured where the rancid butter hit.

"It's clear that the acid was highly concentrated."

Bethune was detained in February after he boarded the Japanese fleet's security ship the Shonan Maru II during its annual cull of the sea mammals.

He was a member of the US-based anti-whaling organisation the Sea Shepherd Society when his powerboat the Ady Gil sank after a collision with the Shonan Maru II, and Bethune wanted to make a citizen's arrest of its captain and charge him for the sunken boat. On trial in Tokyo, Bethune has 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, but he has denied the assault charge.

Japan hunts whales under a loophole to an international moratorium that allows killing of the ocean giants for "scientific research". Meanwhile, anti-whaling group Sea Shepherd says kicking Bethune off its anti-whaling campaign may actually help his court case in Japan.

Closing arguments in the Japanese court case against Bethune are being held in Tokyo District Court and the US-based Sea Shepherd group yesterday defended its expulsion of Bethune, saying the decision upholds a nonviolence policy and may help him gain leniency in court.

The Associated Press reported that Sea Shepherd captain Paul Watson acknowledged the group received some criticism after its decision last week to kick out the New Zealander for having a bow and arrow with him while aboard the Ady Gil, which he said was a violation of policy.

The group, which often has scuffles with whalers, claims that its policy is "aggressive but nonviolent".

Mr Watson said in a statement that the group would continue to help cover Bethune's legal fees for his trial in Tokyo.

Mr Watson also denied the charge by Japanese prosecutors that he had ordered Bethune to climb on to the Shonan Maru 2.

He said the decision to expel him was "taken out of necessity both for Captain Bethune and for the Sea Shepherd Conservation Society".

With Bethune telling the court that he doesn't plan to return to Sea Shepherd's campaigns, the judge will be more likely to release him, Mr Watson said.

"Sea Shepherd is focused on getting Bethune out of a Japanese prison."

Bethune's wife Sharyn earlier this week criticised the Sea Shepherd decision.

Nevertheless, her husband remained positive that he would get a suspended sentence and be out of Japan by early July.

When he got back from Japan he would probably take six months off to finish writing his book and spend some time with his kids, she said.

After that he would probably return to conservation work, "just probably not with the Sea Shepherd", she said.