A cover-up is being alleged over the death of a critically endangered Maui's dolphin outside a protected are off Taranaki, but the Ministry for Primary Industries (MPI) says the claim is unsubstantiated.
German conservation group NABU International spent three years trying to verify claims it had heard one of the dolphins was caught in a gillnet during the 2012/2013 fishing year.
It claims to have a statement from an eyewitness on the boat who said an official government observer told them he had "seen nothing".
There are no recorded deaths of Maui's dolphin in the official database of fishing bycatch for the 2012/2013 year. There are estimated to be around 50 of the mammals left.
NABU head of endangered species conservation Dr Barbara Maas says the group had tried for years to get MPI to confirm the rumour.
"To no avail, and given there are so few animals, we felt compelled to make this information available to the public in the hope it would spark some much-needed change."
She says all those on the vessel saw the dolphin being pulled in by the net.
"From what I understand they were quite distressed about it because they knew they would have to report it and it would be a lot of paperwork."
Dr Maas, who had previously lived in New Zealand and worked for the Department of Conservation in bycatch mitigation, said the Government has a stark choice about their treatment of the rare dolphin.
"There's an anti-science stance that can only lead to their extinction. Either you pull the gillnets or trawl nets out of the area in which the dolphins live, which is what everybody has suggested including us, or Maui's dolphins will go extinct -- that's the choice."
She warns if nothing is done, New Zealand could be on the brink of claiming an unwanted reputation.
"Let's make a science-based decision and protect these animals otherwise New Zealand will be the first to drive a marine cetacean to extinction which is something the whalers haven't managed to do."
Dr Maas says the protection area which covers the dolphins' habitat, mostly around the west coast of the North Island, should be increased.
Greenpeace wants an immediate independent investigation into the claims, saying it is shocked by the allegation.
"It is incredibly serious if, as NABU allege, the Government agency responsible for enforcing the regulations protecting Maui is actually complicit in covering up one of their deaths. This loss would equate to 2 percent of the remaining Maui dolphin global population," New Zealand executive director Russel Norman says.
"It certainly raises questions as to the integrity of New Zealand's system of regulating the fishing industry. It can only be dealt with by an independent investigation into the Ministry."
The Department of Conservation says the dolphin is "on the edge of extinction" and compares it to the kiwi in terms of importance to New Zealand's heritage.
The Government has put set net bans in place including off the coast of Taranaki in an attempt to help save the animals.
But it has also opened up the area to oil and gas drilling which, in 2014, Energy Minister Simon Bridges said was not jeopardising wildlife.
MPI director of fisheries management Dave Turner says they've seen no evidence of Dr Maas' claim and says withholding it is to the detriment of the dolphin.
"If she has any evidence of an incident we ask her to bring it forward so we can investigate. If her allegations were in fact true, that's she's failed to provide the allegations to us immediately puts the dolphins at greater risk by delaying an investigation," Mr Turner says.
Dr Maas made a separate allegation about Maui's dolphin about a year ago which was fully investigated by MPI and was found to be false, Mr Turner says.
He also says her claim between three and five Maui's dolphins die a year "lacks credibility" and is an insult to observers whose job it is to protect the species.
"These people do a remarkable job, a very difficult job, in helping maintain our sustainable fisheries and they don't deserve this type of unsubstantiated allegation."
In around 2000 at-sea observer days since 2012, no observer has witnessed a sighting or capture of a Maui's or the closely related Hector's dolphins.
Throughout the dolphins' known habitat, 8300sqkm of water has been closed to trawl fishing and more than 15,000sqkm closed to set nets.
"We will take any measures that science tells us we need to take to protect them. At times a lot of fishing industry livelihoods have been destroyed through the closures we've made to protect the dolphins.
"I feel for the families who lose their jobs and their income, but at times, for the greater good, those are the calls that we and the government make."