The death of a dolphin after a collision with a boat in the Hauraki Gulf has prompted a warning for boaties to take care around marine mammals.

The 1.7m common dolphin was found at Toroa Point, Torbay, on Sunday with a large depression in its left side and a snapped backbone. Members of the public had found it struggling to swim in shallow waters.

Beachgoers had seen the dolphin swimming in a pod earlier in the day, near to where small motorboats and jetskis were operating.

A post-mortem examination by Massey University marine biologist Karen Stockin found that it was probably paralysed as a result of huge blunt force trauma.

"The animal had a notable impact injury to its lower left side ... A section of its lower spine was completely shattered and extensive internal trauma was evident."

She said most dolphins that came into the harbour to breed or find food were capable of evading larger boats.

However, small speedboats and jetskis that changed direction quickly or travelled at speed could be a hazard for marine mammals.

Dr Stockin urged small boat owners to be vigilant around the many cetaceans found in the gulf's busy waters. Boaties are required by law to slow down around marine mammals and keep at a safe distance.

Auckland University marine biologist Rochelle Constantine, who studies the effect of commercial and recreational boating on cetacean populations, said boat-dolphin collisions were rare.

"But it's not unheard of. At this time of year there's a lot of boats on the water, there's a lot of calves and young animals around, and they're focused on feeding. Dolphins are good at evading boats but they're not perfect."

Dr Constantine said boaties often mistakenly believed they should speed up around dolphins, to encourage them to play, but this raised the chance of harming them.