The total catch between 1950 and 2010 was 38.1 million tonnes, the study says, compared with a reported catch of 14 million tonnes.
The main reasons for this difference were unreported commercial catch and fish which were discarded because they were too small, uneconomic, or because the fisher had no quota.
The findings were revealed in a paper published today, and have prompted a call by environmentalists for an independent inquiry.
Lead researcher Glenn Simmons, from the New Zealand Asia Institute at the University of Auckland Business School, said the paper was part of a project which was designed to make New Zealand's seafood industry as sustainable as possible.
"To maintain sustainable fisheries and seafood businesses, you need to know how much fish is being caught," Dr Simmons said.
"There was already strong evidence that we didn't know that, because the official statistics are incomplete. Unreported catches and dumping not only undermine the sustainability of fisheries, but result in suboptimal use of fishery resources and economic waste of valuable protein."
The paper said that New Zealand's internationally-recognised Quota Management System had incentivised fisheries management by inadvertently rewarding misreporting and dumping.
"A striking finding was the extent of misreporting to avoid deemed value penalties - at sea and on land," Dr Simmons said.
"This highlights a weakness of the QMS, which relies on full and accurate reporting, yet, in practice, incentivises misreporting. Fisheries management and stock assessment officials must spend more time talking and listening to the fishers, observers and compliance officers."
The authors of the paper used Ministry for Primary Industries (MPI) data as a baseline, and supplemented it with stock assessment reports, peer-reviewed studies, data obtained under the Official Information Act, unofficial reports and data from industry experts.
Greenpeace New Zealand wants an independent investigation of the findings, in particular an allegation that the MPI was aware of the systemic dumping of fish but did not act on it.
"New Zealand's industrial fishing companies have been under-reporting the number of fish they have been taking for years, and the evidence suggests the MPI has been covering for them," Greenpeace's executive director Russel Norman said.
The paper quotes an MPI investigation in 2013, which said: "The sight of large, perfectly good fish being systematically discarded in such large quantities could have a huge negative effect, as it could easily stir up an emotive backlash from not only the New Zealand public, but from international quarters as well."
It added: "This combined with the fact that we have known about these dumpings/discarding issues for many years, and would appear to have done little to combat it, would be very difficult to explain and be unpleasant at best."
Ministry concerns
MPI's director fisheries management, Dave Turner, said its initial review of the report has raised concerns about methodology and conclusions.
"We are clear the report simply can't draw adequate conclusions about sustainability, as its authors attempt to, because the measure of sustainability is abundance - that is, the amount of fish in the sea - not extraction as the report attempts to analyse.
"We have decades of peer-reviewed science that shows steadily increasing levels of abundance. The situation now is that New Zealand fisheries are healthy overall, and that's because of careful, science-based management."
The overwhelming majority of fish caught by commercial fishers came from sustainable stocks, Mr Turner said. Rebuilding plans were in place in the remainder of fisheries.
The ministry's initial analysis of misreporting had overstated the issue, he said.
"One serious problem with the report is the way it collects information. Many of the conclusions it draws rely on the recollections of interview subjects."
Meanwhile, Seafood New Zealand chief executive Tim Pankhurst said the report lacked scientific credibility.
"The sample is hopelessly biased. It includes interviews with 300 people, none of whom are named, and 200 of whom were crews on foreign chartered vessels complaining about their treatment.
"The report looks back over 61 years, including the decades when New Zealand had no jurisdiction over catches outside of our Territorial Sea ... it's a whole different ball game now. New Zealand's fisheries management is recognised as one of the best with one of the highest compliance levels in the world."
Key points
• New Zealand's reconstructed marine catch totalled 38.1 million tonnes between 1950 and 2010, which is 2.7 times the 14 million tonnes reported to the United Nations' Food and Agriculture Organisation (FAO)
• Since the Quota Management System (QMS) was introduced in 1986, the total catch is conservatively estimated to be 2.1 times that reported to the FAO
• Unreported commercial catch and discards account for the vast majority of the discrepancy
• Recreational and customary catch was 0.51 million tonnes, or 1.3%
• Only an estimated 42.5% of industrial catch by New Zealand flagged vessels was reported
• Some 42% of the industrial catch was caught by foreign-flagged vessels, which dominated the catching of hoki, squid, jack mackerels, barracoota and southern blue whiting - some of the most misreported and discarded species
- By Isaac Davison of the New Zealand Herald
