An orca hauled up by a fishing trawler last year was probably killed when it was hit by a container ship, an oil tanker or a cruise ship, a report says.
At the time, last year, investigators were unable to determine exactly when the mammal died, or the cause of death.
The female six-metre killer whale was caught in a deep-sea trawler's net in December, about 50km northeast of Banks Peninsula and observed by a fisheries inspector on the boat.
The report said it's the first record of an orca being captured by a trawler and there has only ever been one other New Zealand instance of a fisheries inspector observing an orca capture. This was in 1990 by a Japanese surface longline vessel.
An orca was captured alive in 2003 and released alive, the report said.
A report by Fisheries New Zealand said it was unlikely the trawler hit the orca, as this would have probably been noticed by the crew.
The mammal belonged to the New Zealand coastal population and its condition once on board suggested the killer whale had been dead for some hours before it was hauled up.
There were numerous injuries, but these were inconsistent with having been caused by predators or scavengers.
A long slice in the carcass from its mouth to abdomen was possibly caused by a large vessel's propeller, and another deep cut was also likely caused by a large propeller, the report said.
Wounds on the orca, indicating broken ribs and a broken spine, were probably caused by being hit by a large container vessel or cruise ship.
The report said the most likely cause of death was being struck by a large vessel, which either killed or incapacitated the animal.
"Some shallow lacerations may have been inflicted as the orca's body slid along the side or bottom of the vessel; toward the rear of the vessel the intake from the propeller would have been sufficient to draw the orca's body into the propeller, accounting for the deep longitudinal laceration that also sliced transversely across the orca's abdomen.
"Additional injury may have arisen from subsequent collision with the ship's rudder after passing through the propeller zone.
"After an indeterminate time on the surface, the orca's body most likely sank to the ocean floor, where it was subsequently retrieved in the fishing gear."
The report said in the days before the orca's capture, oil tankers and container ships were crossing that stretch of ocean near a busy shipping lane.