However, the financial damage island states are suffering is less than previously estimated - with improved fisheries policing given some of the credit.
The 17-member Pacific Islands Forum Fisheries Agency (FFA), which New Zealand belongs to, released its study into illegal, unreported and unregulated (IUU) fishing in the region on Tuesday.
The FFA said its “best estimate” was that IUU tuna fishing deprived Pacific nations heavily reliant on rich territorial tuna stocks of more than US$224 million per year.
Overall, Pacific tuna worth more than US$1 billion on the retail market is illegally caught, the report said.
FFA director general James Movick said previous assumptions about who was doing most illegal tuna fishing were incorrect.
“We imagine vast fleets of pirate boats floating around, the evidence doesn't support that.”
The report, Towards the Quantification of Illegal, Unreported and Unregulated (IUU) Fishing in the Pacific Islands Region, said it's likely catch under-reporting is the biggest reason resulting in lower royalties being paid to Pacific nations.
Although Movick called the US$1bn illegal catch “staggering”, he said the report conducted by MRAG Asia Pacific, was a “celebration of effectiveness” of improved fisheries policing.
MRAG Asia Pacific chief executive Duncan Souter said Pacific nations could “take comfort... the scale of the loss is much less than previously thought.”
Previous reports showed illegal fishing meant Pacific countries missing out on up to 20 per cent of royalties, Souter said.
The 2016 FAA report showed that figure had dropped to 12 per cent.
Including non-tuna catch, previous estimates of IUU fishing losses in the Western Pacific Ocean had put the likely cost at between US$1 billion and $US2.4 billion per year, the FFA said.
Tuvalu natural resources minister Elisala Pita said fisheries represented 45 per cent of his tiny country's budget.
Until the latest FFA report, Tuvalu "didn't know how much it was missing out on", Pita said.
“Our marine resources are our main resource, we want to try and stop illegal fishing but this is not an easy thing.”
Movick wouldn't “single out” which fishing fleets, including possibly New Zealand vessels, were likely be to blame for illegal fishing.
However, he said the report findings went some way to reassuring Pacific tuna consumers what they eat was now more likely to come from legal, sustainable fishing.
He hoped the New Zealand government, which is yet to see the report, will maintain its level of fisheries surveillance and monitoring support.
The South Pacific tuna fishery area is nearly one million square kilometres in size.
The report calls for stronger independent catch monitoring from sea to store shelf, better ship to ship cargo monitoring and control, improved shipboard monitoring and the introduction of electronic monitoring.