An undersea volcanic eruption has created a raft of porous volcanic rock in the Pacific Ocean that's larger than the surface area of Israel, but navy officers say the phenomenon is not a danger to shipping.

Pumice is a porous grey-coloured form of volcanic rock formed when lava and water are mixed. Most pumice is light enough to float on water.

The area of floating pumice is 250 nautical miles (463km) in length and 30 nautical miles wide (55km), and covers 25,465 square kilometres.

Israel's surface area is about 20,700 sq km.

Spotted by a Royal New Zealand Air Force Orion, the raft was located about 85 nautical miles west to southwest of Raoul Island and investigated by the HMNZS Canterbury, the New Zealand Defence Force says.

Raoul Island is part of the Kermadec Islands and is 1100km northeast of New Zealand.

Lieutenant Tim Oscar, a Royal Australian Navy officer on the vessel, said on Friday it was "the weirdest 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.

The rock appeared to be sitting above the surface of the waves and when lit up looked like the edge of an ice shelf.

"I knew the pumice was lightweight and posed no danger to the ship. Nonetheless it was quite daunting to be moving toward it at 14 knots.

"As we moved through the raft of pumice we used the spotlights to try and find the edge - but it extended as far as we could see."

The cause is so far unknown.

GNS Science vulcanologist Craig Miller told NZ Newswire the pumice raft has been caused by an undersea eruption and it was originally thought that the undersea volcano Monowai had erupted, but that is northeast of Raoul Island.