An investigation has failed to determine who caused the collision between the Sea Shepherd's anti-whaling speedboat and a Japanese whaling ship in the Southern Ocean earlier this year.

The New Zealand trimaran Ady Gil eventually sank after it collided with the Japanese whaling ship Shonan Maru 2 in the Southern Ocean in early January.

The Ady Gil was sliced in two. Six crew members were almost thrown overboard and one crew member suffered broken ribs.

Both the Japanese whalers and the Sea Shepherd crew blamed each other for the incident.

A report by the Australian Maritime Safety Authority said Japan had refused to answer any questions and seized navigation information from the whaling ship as part of its own inquiry.

Greens Senator Bob Brown says the report is a political cop out.

"This is just a report which is written with an eye to giving the government an out from its responsibility for taking legal action," he said.

"The Ady Gil left Hobart, it was on the high seas, the law is any country can take action against a breach of international law on the high seas."

Japanese and New Zealand authorities are also investigating the collision.