An astonishingly large sea of trash swirling in a centre current of the Pacific Ocean is far more packed with plastic debris than initially estimated, according to a first-ever aerial survey.

The garbage patch between California and Hawaii, which can be seen from space, measures 1.3 million square miles and is filled with chunks of plastics and tangled blobs of fishing line, according to the aerial expedition by the Dutch environmental organisation Ocean Cleanup.

Spotters flying in "Ocean Force One" — a modified C-130 Hercules aircraft — counted more than 1,000 large items of trash in 2.5 hours of surveying the area, the group reported.

The craft is fitted with state-of-the-art sensors that can detect objects at oceanic depths of tens of meters and estimate the size of discovered objects.

"It's worse than we thought," said Ocean Cleanup's 22-year-old founder Boylan Slat at a press conference at Moffett Airfield in Mountain View, California.

Even at the very edge of trash sea, surveyors spotted "more objects than we were expecting to find in the centre" where garbage is more densely packed by the swirling current, he said.

In 2015 Ocean Cleanup sent 30 vessels simultaneously across the Pacific garbage patch to start gathering data to put together a map of the trash sea. The aerial expedition covers much more of the ocean's surface. Flights will continue through 7 October.

The ocean trash breaks down into micro-plastic that is ingested by fish and other marine life that is passed up the food chain.

A 2015 study by scientists from the University of California at Davis and Indonesia's Hasanuddin University found that 25% of fish sold at markets in California and Indonesia contained human-made debris — plastic or fibrous material — in their stomachs.

Ocean Cleanup staffers are writing a research paper on the trash findings and plan next year to test a rubber boom to surround and compress the sea of ocean garbage for collection.

An estimated 269,000 tonnes of plastic are floating in the world's oceans.