WELLINGTON (AFP) – A Japanese whaler did not deliberately ram and sink a Sea Shepherd protest boat during a high-seas confrontation in Antarctic waters earlier this year, a New Zealand report found Thursday.

"The collision appears to have resulted from a failure by both masters and the crew of both vessels to appreciate and react appropriately to the potential for the collision," Maritime New Zealand said in a report on the incident.

The clash on January 6 resulted in the whaler Shonan Maru II slicing off the bow of the Ady Gil, a high-tech trimaran operated by the Sea Shepherd campaign group, which sank two days later.

The Ady Gil had been harassing Japan's whaling fleet as part of Sea Shepherd's campaign to prevent whaling in Antarctic waters.

The militant environmental group had accused the whaler of deliberately crashing into the much smaller protest boat. One of the Ady Gil's crew suffered broken ribs in the collision.

Maritime NZ said both sides "failed to comply with international collision regulations and to act as prudent seafarers should have".

It said the whaler veered towards the Ady Gil before the collision, but the protest boat contributed to the accident by accelerating forward at the last minute in an unsuccessful attempt at evasive action.

The report did not recommend prosecuting the master of either vessel.