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Sensors at the summit of Mt Ruapehu have recorded a series of small earthquakes since Sunday's spectacular lahar.
"The rapid removal of water from above a hydrothermal system can destabilise that system, resulting in small-scale eruptions," said Brad Scott, volcano surveillance co-ordinator at GNS Science, which maintains the
sensor system.
The quakes measured up to magnitude 1.
Lowered lake levels could lead to increased heating and steam-driven eruptions, Mr Scott said.
"If a small eruption occurred under the lake, it could generate waves large enough to overtop the hard rock rim of the lake, resulting in a small lahar."
He said an increase in quakes was common after volcanic lakes emptied and was a sign of changes to the hydrothermal system under the lake.
Mr Scott said hydrothermal eruptions were different from magmatic eruptions such as the 1995 event.
"We're talking about eruptions that may dome up the middle of the lake by 15m to 20m. It's like a geyser."
The chance of another lahar was small but still probable, he said.
An inspection of the Crater Lake by Department of Conservation and GNS scientists yesterday morning confirmed that one part of the tephra dam had collapsed down to the original hard rock rim.
The 7m-high soft rock and ash dam - created by the 1995 eruption and stretching 85m along the crater rim - disintegrated, releasing the pent-up acidic lake water.
As much as 1.3 million cubic metres of water and debris spilled from the Crater Lake and washed down the Whangaehu River to the sea near Wanganui 140km away.
The dam had been weakening under the pressure of the lake and erosion had been detected as early as December.
Some 40mm of rain on Sunday morning is thought to have contributed to the dam's failure.
DoC senior conservation officer Dave Wakelin said the department still considered the Crater Lake a hazardous area but it had no power to bar the public. "All we can do is strongly advise people.
"Nobody's going to arrest you for going to somewhere that's hazardous - but we'd strongly advise you didn't."
Mr Wakelin said water was flowing out of the lake's natural outlet as it had before the 1995 eruption.
"Now you've actually got a lake that finally can just flow in normal fashion."
The Whangaehu River was still dirty with mud and debris but flow levels were returning to normal.
Ontrack spokesman Kevin Ramshaw said a 25km/h restriction remained in place over the Tangiwai rail bridge, which was proving popular with lahar-spotters.
"We've had people not quite by the bus-load, but a large number, queuing up to look at it," he said.
"They should stay away. The railway is not a public thoroughfare."
Ontrack had hired security guards to keep people away from the bridge.
- Additional reporting NZPA
