The New Zealand Government is putting pressure on Japan to participate in an investigation into the sinking of an anti-whaling vessel, Prime Minister John Key revealed this morning.

TVNZ reported yesterday that Japanese authorities were refusing to cooperate with a Maritime New Zealand investigation into the Southern Ocean collision in January between a Japanese whaling boat and the anti-whaling protest vessel Ady Gil. An Australian investigation could not reach conclusions after the Japanese refused to participate.

Protest organisation Sea Shepherd's high-tech powerboat was sliced in two and sank in a collision with Shonan Maru II in January.

"The position from New Zealand's perspective is that Maritime New Zealand undertakes that inquiry," Mr Key said on Breakfast on TV One this morning.

"I have heard, and I do know, that information has been slow but the Foreign Minister Murray McCully has been in touch with his counterparts in Japan and I think he's going to go and ask the Japanese ambassador to come in again this week.

"It is important that all of that information is given to Maritime New Zealand so they can make a fair assessment, as best they can, of what went on."

MNZ spokeswoman Sophie Hazelhurst yesterday would not comment on the New Zealand investigation.

"All we can say at this stage is that our investigation is ongoing."

Ms Hazelhurst said the inquiry still had some way to go, and that the findings would be made public once it was completed.

Mr Key said Ady Gil captain Peter Bethune continued to receive consular assistance while in jail in Japan. Cpt Bethune boarded the Japanese ship before dawn from a jet ski with the stated intention of making a citizen's arrest of Shonan Maru captain Hiroyuki Komiya for what he said was the attempted murder of his six crew.

Cpt Bethune also presented the Japanese whalers with a US$3 million ($4.3 million) bill for his futuristic carbon-and-kevlar trimaran Ady Gil over the January 6 collision.

He faces a trial on charges of trespass, causing injury, vandalism, carrying a knife and obstructing commercial activities.