A marine biologist is urging boaties and jetskiers to be vigilant following the death of a dolphin from an injury likely to have been caused by a collision in Auckland's Hauraki Gulf.

Massey University Institute of Natural Sciences lecturer Karen Stockin carried out a post-mortem yesterday after being called to Toroa Point at Torbay, on the east coast north of Auckland, on Sunday, where a 1.7m dolphin was floundering in the shallows.

Locals reported seeing a pod of dolphins, boats and jet skis in the area just before the dolphin stranded, Dr Stockin said.

The dolphin appeared to have floundered in shallow water and did not seem to be able to swim back out.

"I was called out to assess it but the dolphin had died within a few minutes of stranding," Dr Stockin said.

A post-mortem found it suffered a broken backbone and internal injuries and died of "severe blunt force trauma likely inflicted as a result of a collision with a blunt object".

Dr Stockin, who has researched the New Zealand common dolphin in the Hauraki Gulf for a decade, said they foraged and bred in the gulf and were generally able to avoid boats that followed a predictable course.

Small speedboats and jet skis that changed speed and direction erratically created a hazard for them, she said.

Conservation Department spokesman Phil Brown said the Marine Mammal Protection Act required boaties to keep a certain distance from marine mammals.