A giant pumice raft has been discovered in the Pacific Ocean, near Vava'u, Tonga.

An Australian couple sailing a catamaran towards Fiji were the first to report the raft.

They made the surprising discovery while on a voyage across the Pacific Ocean.

Michael Hoult and Larissa Brill wrote on Facebook: "We entered a total rock rubble slick made up of pumice stones from marble to basketball size.

"The waves were knocked back to almost calm and the boat was slowed to one knot. The rubble slick went as far as we could see in the moonlight and with our spotlight."

Sailor Shannon Lenz also shared footage of the phenomenon online. She wrote: "We sailed through a pumice field for six to eight hours, much of the time there was no visible water.

"It was like ploughing through a field. We figured the pumice was at least six inches thick."

Scientists said the pumice was caused by an eruption of an underwater volcano which erupted off Tonga earlier this month.

The pumice raft is expected to drift towards Australia.