The decision by anti-whaling group Sea Shepherd to ban New Zealander Pete Bethune from future protests was made as an effort to help him avoid jail time, says his wife Sharyn Bethune.

Bethune has been in a Japanese jail since March 12th and is on trial in Tokyo for boarding the Japanese whaling ship, the Shonan Maru 2, is facing up to 15 years jail if found guilty of all charges. His trial will conclude tomorrow with a verdict expected later this month.

Sea Shepherd leadership believed banning Bethune from future expeditions would mean he got a lighter sentence from Japanese courts, Ms Bethune said today.

"I've heard a few opinions that are saying that it's the best move for him and the Japanese might go easy because they know he's not going down to the southern seas and is no longer part of Sea Shepherd," Ms Bethune said.

This view was held by Sea Shepherd's founder and president Paul Watson and deputy chief executive Chuck Swift, she said.

However, she did not believe the tactic was a good one and feared that the prosecution could use the Sea Shepherd ban against him.

"It's very unfortunate, I feel, that it's just before the summing up of the trial," she said.

Sea Shepherd had claimed that Bethune was banned because he brought a bow and arrows on to the Ady Gill, breaking its policy of "aggressive but nonviolent action"

"His decision to bring them on a Sea Shepherd campaign is unacceptable," Mr Swift said in a media release.

Ms Bethune believed Sea Shepherd knew all along that Bethune had brought the weapons on board, saying "they filmed him with them [the bow and arrows] on the boat, so they must have known about it".

There was no intention to fire the arrows at any of the Japanese crew, she added.

Ms Bethune received a letter from Bethune yesterday, she said.

She was unaware whether he knew of the ban but he had already hinted to his daughters that he would not going back to Antarctic waters.

She said Bethune was expecting to be found guilty on three charges but believed the charge of assault would not stick.

He was unhappy with the way the defence lawyers were "almost bowing down and rolling over for the prosecution".

"I know Pete is not happy with the way the defence team are dealing with it, that they are not aggressive or strong enough in his favour," Ms Bethune said.

"Pete's remaining positive that he will 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.

She was fine with him continuing to protest "as long as he doesn't do anything silly."

Bethune has admitted to four charges in relation to boarding the Shonan Maru.

He has pleaded not guilty to one charge of assault and faces up to 15 years in prison if convicted of all charges.

After three days of hearing in Tokyo District Court last month, closing arguments will be heard tomorrow.