NASA has captured satellite images of a giant pumice raft drifting near Tonga, revealing the rubble slick was once roughly the size of Manhattan, in New York City.

Key points: The raft has been dispersing since a volcanic eruption, believed to have occurred near Tonga, in early August

The raft has been dispersing since a volcanic eruption, believed to have occurred near Tonga, in early August A Smithsonian Global Volcanism Program report said the slick was 50 kilometres long at one point

A Smithsonian Global Volcanism Program report said the slick was 50 kilometres long at one point NASA's last observation of the raft said it had broken up further but was still visible

The floating mass is most likely the result of an eruption from an unlisted submarine volcanic vent near the island of Late on August 6 or 7, according to a report on the Volcano Discovery website.

Lava from this eruption formed into vast amounts of porous rock, floating on the surface of the ocean in a gigantic clump known as a pumice raft.

A Smithsonian Global Volcanism Program report said the surface pumice layer was as deep as 30 centimetres, according to an observation on August 9, with pieces as large as 80 centimetres in diameter.

On August 11, the report said the raft averaged a size of about 2.5 kilometres wide, stretching across 35 kilometres.

Another band spanned for as long as 50 kilometres, the Smithsonian site reported.

NASA Earth Observatory compared the raft to Manhattan. ( NASA Earth Observatory )

NASA's Operational Land Imager captured a photo of the rubble slick on August 13, when it had drifted south-west from the site of the volcano.

At this point, the raft of rocks had gathered in a clump roughly the size of Manhattan, which has an area of nearly 60 square kilometres.

Two days later, Australian couple Michael Hoult and Larissa Brill's catamaran passed through the rubble.

As they approached, they noted the distinct smell of sulphur.

"The waves were knocked back to almost calm and the boat was slowed to 1 [knot]," the pair wrote in a Facebook post.

"The rubble slick went as far as we could see in the moonlight and with our spotlight."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 1 m Michael Hoult and Larissa Brill sailed through the rubble slick on while travelling to Fiji.

After clearing the raft, the pair noted a few paint chips but wrote on an online yacht tracking platform that their vessel was "in pretty good shape".

"I was worried there might be some larger boulders in there, fridge or car size, that could do serious damage," the post read.

"The hull has never been cleaner.

"It's literally been exfoliated."

Queensland University of Technology geologist Scott Bryan said pieces of pumice from the eruption would wash up on Australian shores in seven to 12 months.

"When it gets here, [the pumice raft will be] covered in a whole range of organisms of algae and barnacles and corals and crabs and snails and worms," he said.

"We're going to have millions of individual corals and lots of other organisms all coming in together with the potential of finding new homes along our coastline."

NASA's last mention of the raft was on August 22, noting it had moved north.

It has broken up further but was still visible, according to a NASA blog post.