The dolphin is believed be a critically endangered Maui's dolphin - one of only just 55 adults of its kind.
A recreational fisher found it on Friday at Glinks Gully, 15 kilometres south of Dargaville, inside the North Island West Coast Marine Mammal Sanctuary preamp
An initial report from Massey scientists found early indications are that the dolphin was an elderly female who died of natural causes, a spokeswoman for Conservation Minister Nick Smith says.
The dolphin is believed to be a Maui dolphin, because it was found where Maui's dolphins hang out, but DNA testing would be carried out to confirm this, she told NZ Newswire.
Department of Conservation staff took the dolphin's carcass to Massey University's specialist marine mammal unit in Palmerston North, who are investigating the dolphins death.
The death has concerned conservationists with so few Maui's dolphins remaining.
"With an estimated population of just 55 adults, the loss of even one of these mammals puts at further risk the survival of their species," Dr Smith said custom embroidery
The government will be finalising a threat management plan for Maui's dolphins later this year.
This will follow a consultation period on a proposal to extend the set net fishing ban area by 350sq km off the coast of Taranaki, at the extremity of the Maui's dolphin range.
However, WWF NZ and Forest and Bird say the extension does not go far enough to save the species.
WWF is calling for harmful fishing practices to be prohibited and a moratorium on risky marine mining activities from Maunganui Bluff to the Whanganui river mouth out to 100 metres deep Ergonomic furniture