A dead orca has washed up on a West Auckland beach.

Orca are considered nationally critical in New Zealand, and known threats include interactions with fisheries and boat strikes. The death is being investigated.

The orca was discovered on Whatipu Beach, 40km west of downtown Auckland, and reported to the Department of Conservation (DoC). Photos taken by rangers show possible blunt force trauma to the head.

A team from Massey University will today do a necropsy to establish cause of death, and also take biological sampling to assess diet and pollutant loads in the adult male whale.

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The team will be led by Coastal-Marine Research group director, Dr Karen Stockin, who said it was too early to speculate on the cause of the head trauma, although boat strike was an obvious consideration.

Whale Rescue co-founder Jo Halliday said orca around New Zealand faced a higher risk of stranding than those elswhere in the world because of a tendency to chase food into shallow water.

"It can put them at a much higher risk of stranding than other orca that are found in other populations, just because these guys will spend quite a bit of time getting into the shallows chasing after stingrays."

Halliday also said there was no clear indication as to what had caused the death.

"It could be that he may have been stranded somewhere or it could be that it's sick, who knows?"

According to the Department of Conservation, threats facing orca around New Zealand include disturbance caused by boats, which are known to disrupt the animals' normal behaviour, particularly resting, and underwater noise may disrupt echolocation signals and other communication.

As orca were at the top of the food web, they were particularly susceptible to pollution via bioaccumulation - the accumulation of toxins through the food chain.

Marine mammals in New Zealand are legally protected under the Marine Mammal Protection Act and anyone who accidentally kills or injures one must report it to a fishery officer or DoC within 48 hours.

The Marine Mammal Protection Regulations covered commercial whale- and dolphin-watching activities, and incidental recreational interaction.

Under these regulations vessel skippers must avoid rapid changes in speed and direction and not exceed speeds faster than the slowest mammal within 300m.