A female dolphin carrying a dead calf has been spotted swimming in Whangarei Harbour.

Northland orca researcher and whale expert Ingrid Visser was on the water last Thursday and managed to take photos of the female dolphin, known as Potato, with the dead calf.

"The dead baby was very decomposed, most bones gone, blubber floating at the surface when she stopped carrying it, for moments only. She was carrying it on her dorsal fin, across her pectoral fins and across her tail flukes," Dr Visser said.

The dolphin managed to travel up to 8 knots and went to the bow of Dr Visser's boat at one stage.

"There is concern for her in terms of the decomposition of the carcass - having it near her eyes, mouth and blowhole when it is full of bacteria is not a good option."

Dr Visser said they could only get close during the two hours they tracked the dolphin.

The bond between female dolphins and their calves is known to be very strong.

It is not know whether the dead calf was Potato's or not.

Bay of Islands dolphin guide Jo Halliday said Potato was well known and was spotted in the Bay in December.

Ms Halliday, who works on the Fullers Great Sites boat Tutunui, said dolphins grieved over the loss of a calf and while it was not uncommon for them to carry decaying offspring, actually seeing them was heart-wrenching.

"This behaviour is part of their grieving process and it's their emotional side coming out."

Ms Halliday said the gestation period for a dolphin was up to 11 months and once born the group would look after the calf.

A calf could suckle its mother for up to three years.