A navy ship heading to the Kermadec Islands has sailed into a huge 25,000 square kilometre area of pumice pieces north of Auckland.



It is believed to be from New Zealand's third erupting volcano – the undersea mount Monowai. In the past week both Mt Tongariro and White Island have erupted.



The navy said the raft – 463 kilometres by 55 kilometres – was spotted by an RNZAF Orion returning on patrol from Samoa.

Lieutenant Tim Oscar, a Royal Australian Navy officer on an exchange with the NZ Navy, saw what he described as "the wierdest thing I've seen in 18 years at sea."

"The lookout reported a shadow on the ocean ahead of us so I ordered the ship's spotlight to be trained on the area. As far ahead as I could observe was a raft of pumice moving up and down with the swell," he said.

Navy officers with a piece of the pumice.

"The rock looked to be sitting two feet above the surface of the waves, and lit up a brilliant white colour in the spotlight. It looked exactly like the edge of an ice shelf."



Canterbury, which left Auckland on Wednesday, sailed to the raft to pick up a sample, which would be analysed to determine which volcano they came from.

A science writer on the voyage to the Kermadecs has been keeping a journal of findings from each day.

Rebecca Priestley said it was "an event" which caused the Canterbury's Commanding Officer, Commander Sean Stewart to give the order to change course.

"Up to 250 nautical miles long by 30 nautical miles wide, it stood out against the blue-grey of the ocean as a great white froth on the surface of the sea," Priestley wrote.

She said they came across it about midday yesterday.

Navy ratings reportedly lowered buckets, tied to a rope, off the gun deck and down into the water to collect deposits, which Marine Geologist Helen Bostock, who is also on the voyage, would take back to Niwa to examine, Priestly said.

GNS scientists are aboard the ship, which is also carrying 30 high school students on a Sir Peter Blake fellowship to Raoul Island.

GNS vulcanologist Craig Miller said they were aware of the ''pumice raft'' but did not know the exact dimensions of it.



He said it was difficult to guess how big the pumice raft could be, but the air force had flown over and assessed its size.



"We've been in contact with the air force recently about it. But it is floating more than 1000 kilometres offshore, so it's a while away."



Miller said the pumice raft was about "half way to Tonga", and just past Raoul Island.