New Zealand is one of only two countries to vote against further protection for the critically endangered Maui's dolphins at the world's largest conservation summit.
A motion, aimed at stopping the extinction of the world's rarest dolphins and porpoises, including New Zealand's Hector's and Maui's dolphins, was put forward at the IUCN's World Conservation Congress in Korea last week.
Some 576 members, including governments and NGOs, voted for the motion, while only two - one of which was New Zealand - voted against.
Conservation groups said the Government's move had tarnished the country's reputation on the world stage.
"The New Zealand Government has acted shamefully and can no longer claim to be leaders in conservation," WWF's marine programme manager Rebecca Bird said.
"If we fail to act now, it will not be long before Maui's disappear from our waters forever."
The Government's vote was revealed only after a Department of Conservation staff member told the congress that the Ministry for Primary Industries had instructed it to vote that way, Radio New Zealand reported.
The motion included to urgently extend protection measures, with an emphasis on banning gill net and trawl net use from the shoreline to the 100 metre depth contour in all areas where Hector's and Maui's dolphins are found.
In a statement to IUCN member, the Government said action had already been taken to protect the endangered dolphins, and the recommendation was not backed by scientific evidence.
Barry Weeber, co-chairman of the Environment and Conservation Organisations, which sponsored the motion, said "almost everyone apart from the New Zealand Government wanted to see a strong resolution".
He said the Government had expressed concern about the resolution before the vote, and a couple of changes had been made as a result.
"We thought that those changes would be sufficient to get New Zealand to abstain or vote for the resolution.
"We did know that they had to go back to Wellington to confirm the changes to see if they were happy with that, which clearly they weren't."
Greenpeace Oceans Campaigner Karli Thomas said the Government had "arrogantly dismissed" international concern about endangered dolphins and "severely tarnished" the country's global reputation.
"Our Government is letting minority business interests ride rough shod over the values of ordinary New Zealanders," he said.