New Zealand anti-whaling protester Peter Bethune will give evidence in Tokyo today, the last day of a trial which could see him facing up to 15 years in prison if found guilty.

Bethune ,45, from Auckland, is expected to argue that he has done nothing illegal, and raise the issue of illegal whaling, his lawyer Dan Harris said at the weekend.

Bethune, a member of the Sea Shepherd Conservation Society, is being tried on five criminal charges, three related to boarding the Japanese whaling fleet's security vessel Shonan Maru 2 in the Southern Ocean on February 15.

He has conceded four of the charges but has contested an assault charge over allegedly hurling a bottle containing butyric acid, or rancid butter, on to the Shonan Maru 2 and injuring a crew member on February 11, as the protesters tried to disrupt the whaling activities.

He boarded the Shonan Maru 2 and presented its captain with a letter seeking $3 million in replacement costs for the trimaran Ady Gil, which he captained before it sank after a collision with the Shonan Maru 2.

Mr Harris said he was confident that the charge of assault would not be taken seriously by the court, as nobody was badly injured and the bottle had not been aimed at anyone.

A small but noisy group of nationalist protesters has been demonstrating outside the three-day trial in Tokyo District Court. A verdict is expected in late June.