Undersea volcano nearby New Zealand creates 25,000sq km pumice 'island', now floating in the south pacific.

A mass of small volcanic rocks nearly the size of Belgium has been discovered floating off the coast.

The stretch of golf-ball-size pumice rocks was first spotted this week by a New Zealand air force plane about 1,000 kilometres northwest of Auckland.

The rocks stretch for about 26,000 square kilometres.

A navy ship took scientists to the rocks Thursday night. Naval Lt. Tim Oscar says the rocks appeared a brilliant white under a spotlight, like a giant ice shelf.

He says it's the "weirdest thing'' he's seen in 18 years at sea.

"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," he said.

Lt. Oscar said he had been briefed by GNS Volcanologist Helen Bostock the previous day when the ship first encountered an area of pumice from an undersea volcano.

"I knew the pumice was lightweight and posed no danger to the ship. None the less it was quite daunting to be moving toward it at 14 knots. It took about 3 - 4 minutes to travel through the raft of pumice and as predicted there was no damage. 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."

Scientists say the rocks likely spewed up in an eruption by an underwater volcano. They don't believe the eruption is connected to the onshore ash eruption this week of another volcano, Mount Tongariro.

Officials say the small rocks pose no danger to shipping.

The Defence Force says the mass of rocks stretches 250 nautical miles by 30 nautical miles.