New Zealand anti-whaling activist Peter Bethune is likely to plead guilty to four of the five charges laid against him when he goes on trial in Tokyo District Court tomorrow.

Bethune, 45, in Auckland, is expected to admit trespass, possession of a weapon, damage to property and obstructing commercial activity, but deny the most serious charge of assault.

Bethune was working for the US-based Sea Shepherd Conservation Society, on its the Ady Gil trimaran - formerly the record-setting Earthrace--- when it and the Japanese whaling fleet's security ship the Shonan Maru II collided in January.

He later boarded the vessel from a jetski to make a "citizen's arrest" of its captain for what he said was the attempted murder of the Ady Gil's six crew, carrying a knife to cut netting on the vessel.

He was detained and taken back to Japan.

His US lawyer Dan Harris has criticised the Japanese authorities for trying to stage "a political show trial".

Sharyn Bethune, 41, of Auckland, told 3News tonight that her husband would deny the assault charge - which carries a maximum penalty of up to 15 years - and had written a speech in Japanese to help his case.

"Pete's got to be humble, he's got to be apologetic, Pete will find that hard if he believes that he's in the right," she said.

He said in a letter home that "the best I can hope for is a suspended sentence".

"The lawyers believe I will get this if I put on a good but humble show at the trial".

But he noted: "Humble is not so easy though when you've had your boat sunk and been locked up for months.

"If I get a suspended sentence and leave here June or early July, I'll settle for that".

Mrs Bethune said her family would not attend the trial. "He'll be in chains probably on the dock - not an ideal thing for the kids to see".

Instead she'll visit him with their two teenage girls daughters Alycia, 13, and Danielle, 15, in July if the court hands down a long sentence.

The Green Party today criticised the Government for failing to ensure the completion of a Maritime New Zealand report on the sinking in time for use by defence lawyers at the trial.

"The Maritime New Zealand investigation into the alleged ramming of the Ady Gil is critical to Pete's defence, and yet it remains incomplete and secret after all these months," Green MP Gareth Hughes said.

Prime Minister John Key has said the maritime report on the sinking of the Ady Gil required input from Japan, which had been slow in coming.

Mr Key said Bethune was getting support from New Zealand diplomats and there would be an official presence during the trial.

- NZPA