Population numbers of the Maui's dolphin, the world's smallest and scarcest sub-species found only in waters off New Zealand's North Island, have dwindled to less than 50, prompting fears it could reach the point of no return without strong action, including a ban on fishing in its habitat.
The IWC, in a report released over the weekend, said not enough was being done to prevent the dolphins becoming "bycatch" to the commercial fishing industry.
"Existing management measures in relation to bycatch mitigation fall short of what has been recommended previously."
The IWC said Wellington had been saying since 2013 that it was researching the issue, but it had not taken any steps to rectify it.
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Barbara Maas, an endangered species specialist with conservation group NABU International, said the New Zealand government had repeatedly ignored the IWC's concerns.
"The science is clear, if nothing is done then the Maui's dolphin is gone," she told AFP.
"What's needed is action, not more research. This is a huge indictment on New Zealand."
She has called for a boycott of New Zealand's billion-dollar fishing industry, arguing it is the only way to force action on the issue.
Greenpeace said New Zealand could not continue to trade on its "clean, green" image while allowing the Maui's dolphin to edge towards oblivion.
"How many damning scientific reports do we need to see before the government agency actually does something," Greenpeace NZ executive director Russel Norman said.
"Protecting New Zealand dolphins from extinction is part of what it means to love this country. We market this country on swimming with dolphins."