A huge pumice raft spewed up by an undersea New Zealand volcano has been captured by a Nasa satellite.

It reveals that when it erupted on July 19 it even sent up a plume of ash that went unnoticed.

The first sign of it came last week when a Royal New Zealand Air Force Orion patrol spotted the pumice and re-rerouted multi-purpose warship HMNZS Canterbury to the scene, south of the Kermadec Islands, last Friday.

They found thick pumice for as far as the eye could see.

After reading the report, Nasa searched their database of satellite shots - coming up with these.

Scientists from Tahiti and New Zealand's GNS Science connected the eruption with a cluster of earthquakes in the Kermadec Islands on July 17 and 18.

Working independently of GNS, vulcanologist Erik Klemetti and Nasa visualiser Robert Simmon examined a month's worth of satellite imagery from Nasa's Moderate Resolution Imaging Spectroradiometer.

They discovered the first signs of the eruption - ash-stained water, gray pumice, and a volcanic plume - in imagery from 9.50 am and 2.10 pm on July 19.

"Hidden by clouds in the morning image, the site of the eruption is clearly visible in the afternoon image," Nasa says.

They then identified the likely source of the eruption as the Havre Seamount.

The eruption was strong enough to breach the ocean surface from a depth of 1100 metres.

By July 21, the eruption appeared to have waned, leaving behind the dense rafts of pumice.

- © Fairfax NZ News