The Great Pacific Garbage Patch could be up to 16 times larger than previously believed, scientists say.
There are an estimated 1.8 trillion pieces of rubbish floating in waters between Hawaii and California, new research has found.
The latest air surveys of the region show plastic covers at least 1.6 million square kilometres - an area six times larger than New Zealand, and a minimum four times larger than previous estimates. The total would weigh between 49,000 and 129,000 tonnes.
Around half of it is plastic fishing nets by mass. Other items the researchers from New Zealand and the Netherlands found included plastic bottles, lids, ropes and packaging straps. Some of it dated back to the 1970s.
While around 94 percent of the 1.8 trillion pieces are estimated to be microplastics, their small size means they make up only 8 percent of the total mass. But it's increasing - in the 1970s it's estimated there was around 0.4kg of microplastics per square kilometre of the Great Pacific Garbage Patch - now it's 1.23kg.
- Scientists propose a ban on glitter
- Microplastics 'ubiquitous' in bottled water - report
- 5.25 trillion pieces of plastic litter Earth's oceans
All up, 99.9 percent of the patch is plastics.
Previous estimates of the size of the patch were too small, the researchers say, because they "used only vessel-based visual surveys".
"The differences between estimates could also be attributed to increasing levels of ocean plastic pollution in the area in the time since the previous studies were carried out, particularly following the 2011 Tohoku tsunami," they said in a statement.
The problem could even be a lot worse than aerial surveys suggest.
"Only certain types of debris that were thick enough to be buoyant stayed and accumulated in this zone, such as the common plastics polyethylene and polypropylene, which are used in packaging," they said, adding that their estimate of the total amount of plastic is "conservative".
"Our results suggest that ocean plastic pollution within the Great Pacific Garbage Patch is increasing exponentially."
- How one plastic bag can harm millions of creatures
- Government rubbishes National's concern over ocean plastic
A study released a week ago found many popular brands of bottled water contain microplastics.
Research last year estimated there were more than 5 trillion pieces in the oceans worldwide - about 60 percent of it from China, Indonesia, Philippines, Vietnam and Thailand.
The latest research on the Great Pacific Garbage Patch was carried out by The Ocean Cleanup Foundation in the Netherlands and The Modelling House in Raglan. The study was published in journal Scientific Reports.