THE largest pumice raft seen in the past 50 years has arrived on the northern Great Barrier Reef from an underwater volcanic eruption off NZ.

Pumice is the light and porous rock found on beaches. It forms when frothy lava solidifies.

The NZ eruption formed a 20,000 sq km raft that eventually spread about 4 million sq km as it broke up.

Queensland University of Technology researcher Scott Bryan said it was the largest pumice raft in the past 25 to 50 years, eclipsing even that caused when Krakatoa in Indonesia erupted in 1883.

A section of the raft covering more than half a sq km has been recorded off the Low Isles northeast of Cairns by island caretaker Wayne Fox.

Dr Bryan said the pumice was teeming with marine hitchhikers, including barnacles, molluscs, anemones, bristle worms, hydroids and crabs and showed how places such as the Reef came to have so many species.

The raft had travelled about 4000km and some of its pieces were about the size of a human head.

Dr Bryan said the pumice was the result of an eruption by the Havre Seamount in July last year in the Kermadac Islands, north of NZ.

The pumice mass had broken up and spread from NZ to Tonga and from northern NSW to north of Cairns. In some places it was tens of centimetres deep.

Where it went next would depend on winds and currents, which had the potential to send it both south and north.

Dr Bryan said such rafts were the major way through history and evolution that species had been transported.

It was likely the Great Barrier Reef obtained its vast array of species from these events, rather than itself being a cradle of diversity.

"Pumice rafts are the only process in evolutionary history that can transport species fairly rapidly - up to 30 kilometres per day - across deep oceans that would normally act as geographic barriers," Dr Bryan said.

"In the past, we've seen rafts become home to micro-communities of more than 80 species including corals, and sometimes the weight of the hitchhikers is so great that it causes the pumice to sink."

When Krakatoa erupted about 36,000 people died mostly from tsunamis. Human skeletons were found on pumice rafts in the Indian Ocean up to a year later.

Dr Bryan said the raft had previously been seen off southeast Queensland, northern NSW and Heron Island.

"Rather than coming straight up the coast, some of the species on the pumice indicate that this raft may have taken a different path," he said.

"It's like a big jigsaw puzzle. As we get more reports, we can put the pieces together to tell the whole story."

The Seamount eruption went unnoticed for two weeks until a keen-eyed tourist flying back to NZ from Samoa saw the pumice from an aircraft.