A new Department of Conservation study shows there have been 35,000 unexplained deaths in the past 11 years of the already critically endangered antipodean albatross.
Conservationists blamed the deaths on the longlines used by fishing boats, which the birds got caught in, and said if things did not change, the birds would be extinct within the next 20 years.
Sea bird expert Dr Stephanie Borrelle, from BirdLife International, said at the same time as their numbers plummeted, the number of observers on New Zealand longline boats had continued to fall.
Last year just 8 percent of longline hooks were observed by Fisheries New Zealand, compared to 25 percent in 2015.
"New Zealand needs to lead by example because we can't tell other countries to increase observer coverage or use mitigation measures if we're not doing it ourselves, and we can't prove that we're doing it ourselves."
She said most of the boats came from China, which like New Zealand was subject to international regulations.
While New Zealand vessels undertook to use mitigation measures such as bird scaring devices, having their baits drop faster and fishing at night, she said there was a real need to place cameras on boats to ensure crews were doing what they said they were doing.
She said in Australia cameras had already made a huge difference.
"They've seen their compliance increase really drastically over the past few years and also their seabird captures have gone down, and they're quite low now."
Borrelle said the high number of breeding females killed in longlines was a particular problem for the albatrosses as the males mated for life with one bird and were unlikely to find another if the female died.
She said time was running out for antipodean albatrosses unless longline fishers changed their ways.
"They'll be functionally extinct in 20 years ... so we don't have much time."
Forest and Bird's Kevin Hague said by-catch was the single greatest threat to the antipodean albatross.
"We are very pleased that as a result of Forest and Bird pressure the government has adopted a national plan of action for sea birds that sets zero by catch as a goal of the policy, but what we now need urgently for the antipodean albatross is action to ensure that that goal gets translated into doing something quite different at sea."
He said a fall in observer coverage on New Zealand longline boats, down by two thirds since 2015, was inexcusable.
"At a time when actually this species needs us to know more about what's going on at sea, we've been getting less and less observer coverage."
Fisheries New Zealand said observer coverage on boats could fluctuate depending on other monitoring priorities, fishing activity and the weather.
Sector group, Seafood New Zealand said the industry was committed to mitigating its impact on seabirds and had made significant progress in doing so through a number of trials of new technologies.
It said it was willing to work with the government on the rollout of onboard cameras.