KEY POINTS:
The lowered crater lake at Mt Ruapehu is now at a higher risk of hydrothermal "steam-driven" eruptions, GNS Science experts say.
A lahar at the lake yesterday released 1.3 million cubic metres of water and debris which had accumulated behind a dam of soft rock built up by the 1995 volcanic eruptions.
That reduced the lake level by about 6m and nearby roads and railways were closed while the lahar emptied into Whangaehu River.
Nobody was injured and only minor damage to property was reported.
The dam of soft rock was completely washed away, meaning no more lahars like yesterday's were now expected.
GNS Science volcano surveillance co-ordinator Brad Scott said the drop in the volume of water in the lake meant the balance between water and steam had changed.
"The relationship between the water and steam is controlled by the weight of the water, and what you've done at Ruapehu is lightened the load (of water) and that allows more steam to be produced.
"When you produce more steam it adds more energy to the water, and the steam will try to pass through to the surface and if you end up with excess steam you could end up with what we call a hydrothermal, steam-driven eruption."
Mr Scott said hydrothermal eruptions were different to magmatic eruptions like the 1995 event.
"We're talking about eruptions that may dome up the middle of the lake by 15 to 20 metres. It's like a geyser."
Geysers and waves in the lake could also cause small lahars, if water spilled over the crater rim, he told NZPA.
Hydrothermal activity presented a danger to the public and GNS would work with the Department of Conservation (DOC) to warn people away from the crater if there was an imminent threat .
Two DOC staff members and two GNS scientists were helicoptered to the summit this morning to assess the changes to the lake .
DOC senior conservation officer Dave Wakelin said the department still considered the crater lake a hazardous area, but DOC had no power to bar the public from the a rea.
"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."
He said Whangaehu River was still dirty with mud and debris, but flow levels were returning to normal.
Ontrack spokesman Kevin Ramshaw said this afternoon a 25kmh speed 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 go look at it. They should stay away; the railway is not a public thoroughfare."
Several people had made their way on to the bridge itself, he said.
Ontrack had hired security guards to keep people away from the bridge and they were expected to arrive tonight or first thing this morning.
How long the bridge would be guarded had not been decided, Mr Ramshaw said.
"We'll make a judgement call -- after two or three days presumably the place will lose its appeal and we can stand the guards down ."
- NZPA
