Pumice found washing up on Northland's east coast beaches in recent weeks is almost certainly from an enormous underwater eruption near the Kermadec Islands last year.
No one knew about the eruption of the Havre Seamount in July 2012 until a passenger on a commercial flight between Samoa and Auckland noticed a vast raft of pumice floating on the ocean surface about 600km north of Cape Reinga.
It was subsequently spotted by a New Zealand Defence Force Orion and the Navy ship HMNZS Canterbury sailed right through it.
The floating mass measured 460km long by 55km wide and covered an area of about 25,000sq km - making it almost twice the size of Northland.
In recent weeks, Russell charter boat operator Stephen Western has been finding large quantities of pumice on Bay of Islands beaches, including on Urupukapuka Island, Motuarohia/Roberton Island and at Russell. Mr Western suspected the pumice came from last year's undersea eruption and vulcanologist Brad Scott, of the GNS Wairakei Research Centre, agreed.
Mr Scott said pumice had started turning up on Tongan beaches in November, but prevailing currents could also take it in the opposite direction to New Zealand.
The porous rock could turn up on beaches if floods eroded old pumice beds and washed it downstream, but Northland had little in the way of thick pumice beds.
GNS scientists had taken samples of the pumice raft during an expedition to Raoul Island. Chemical tests could prove the origin of the Northland pumice but were expensive to carry out, so would be done only if there was a compelling reason.
One clue to look out for was the shape of the pumice, Mr Scott said. Freshly erupted pumice would still be rough and angular, while pumice which had been in the sea for a long time was smooth and rounded.