Takerei Gallagher closes the road at the corner of SH1 near National Park.

The view towards Ruapehu and Tongariro from Waiouru.

Ash on the road from the Tongariro eruption.

Authorities brief the media about the eruption.

The view of Mount Tongariro from Lake Rotoaira, on Sh46.

Ash led to all flights to and from the Napier-Hastings' airport being cancelled.

Josh Martin, 14, and his mother Karen Martin cleaning up a friends' property after Mt Tongariro erupted for the first time in over 100 years.

A motorist is surrounded by a cloud of ash on SH1 after Mt Tongariro erupted for the first time in over 100 years.

Road signs and the surrounding area are coated in ash on SH1 one after Mt Tongariro erupted for the first time in over 100 years.

Road signs are coated in ash on state highway one after Mt Tongariro erupted for the first time in over 100 years.

A sheep is covered in ash after Mt Tongariro erupted for the first time in over 100 years.

A pair of glasses are covered in ash after Mt Tongariro erupted for the first time in over 100 years.

Karen Martin helps clean up ash at a friends' property after Mt Tongariro erupted for the first time in over 100 years.

Boulders gouged out metre sized 'impact' craters along the Tongariro Alpine Crossing track near Ketetahi Hut on Mt Tongariro.

Ash covering rock close to the Tongariro Crossing track from the ash cloud from the Tongariro eruption.

Ash blankets the slopes below the Te Maari crater on Tongariro, two days after the eruption.

The weather closes in on Tongariro as DOC and science crews fly around the crater to get a better view, two days after the eruption.

A flyover of Mt Tongariro is showing some vents on the mountain are still steaming, but activity levels were low say GNS Science.

The Crown Research Institute said the observations did not change the volcano's prognosis for eruption though.

"Current eruptive activity is low level but could re-commence at any time. The Aviation Colour Code is reduced to Yellow, which recognises that activity has decreased but renewed unrest is possible."

VOLCANO: White Island.

It was hard to tell what was happening above the Upper Te Mari crater, because cloud cover was blocking views into the vents.

GNS said previously steaming ground at Ketetahi and the Lower Te Mari crater appeared "more vigorous", but there were no obvious major changes.

Vulcanologist Michael Rosenberg said the flyover showed blocks of old lava and hydrothermally altered lava up to about one metre size were ejected by Monday night's eruption.

The surrounding area shows the battle scars of Monday night's eruption with volcanic rock and ash clearly visible down to ground level.

Residents around the base of Mt Tongariro remain in a "state of readiness" should the crater suddenly burst into life.

But the small scale eruption has not deterred thrillseekers heading up to Ruapehu ski fields.

They included young Dannevirke couple Phillip and Amanda Naus who are on a week's holiday in the area with friend Marcel Henman from Wellington.

"To be honest we are not sure what all the fuss is about," Phillip Naus said.

"We can't see any issues around the ski fields and we have been staying in Kuratau around by Turangi and we never even heard the bloody thing on Monday night."

"I honestly think its a big drama over nothing," he said.

The first the trio heard of the eruption was a text message from the other side of the world.

"My father is in America and he text me to see if we were all ok - how weird is that," Amanda Naus said.

"We haven't seen any ash, we haven't even smelt the sulphur - so how are they suddenly smelling it down in Wellington when we are right here," said Henman.

Tourists like Hiro Yashima from Japan were also heading to the skifields, unfazed by the acitivity reported on the neighbouring peak.

"I heard that the mountain went boom, so I'm ready with my camera in case it goes boom again," he smiled.

HAS WHITE ISLAND ERUPTED AGAIN?

Meanwhile, scientists were scrambling to make sense of a number of signs which may indicate White Island has also erupted.

Almost 24 hours after Mt Tongariro blew a new hole in its side, GNS vulcanologists said a tremor "dropped sharply" overnight at the active volcano off the coast of Whakatane.

Craig Miller said the tremor at about 11.30pm was followed by another short burst of activity about 3am.

"The Crater Rim camera appears to be splattered with mud/ash this morning so it seems there has been some kind of burp.

"The plume out of the crater lake from the Factory camera also appears darker and maybe more ash rich."

Miller said it appeared the nature of activity at White Island might be "changing somewhat".

White Island is one of New Zealand's most active cones. Its volcanic alert level sits at two - meaning there has been "minor volcanic activity".

An aviation warning is at code orange, warning pilots that the volcano is "exhibiting heightened unrest" with an increased likelihood of eruption.

Sitting about 48km off the coast of Whakatane, White Island has been created by continuous volcanic activity over the past 150,000 years. About 70 per cent of the volcano sits underneath the sea, making it one of the biggest volcanoes in New Zealand.

SURPRISING SMELLS

Gas and sulphur smells from the Mt Tongariro eruption have travelled as far as Wellington this morning.

The crater spewed rocks and ash when it came to life for the first time in more than 100 years at 11.50pm on Monday night.

Many people have reported a smell of sulphur in the air in the capital.

Miller said it was most likely the smells were caused by the eruption and there had been quite a few reports from the lower North Island of a sulphur smell.

"It's a little bit surprising it has gone as far as it has. It will be dissipating as it goes."

Hutt Valley resident Heidi Parker said she thought the smell was just her damp front yard at first but she could smell it all the way to work in Newtown. She said her colleagues had noticed it too.

Titahi Bay resident Robert Gibson said he and his colleagues at work had noticed the smell and were using air freshener to deal with it.

"It was a sulphury sewage-like smell. I thought my drains were blocked at first."

MetService forecaster Micky Malivuk said wind conditions were right for the smell to travel south.

Northerly winds of between 20kmh and 50kmh had been prevailing since yesterday afternoon.

The smell of sulphur was also strong around Wairarapa but only faint in Hawke's Bay.

Horizons Regional Council said it had received reports of sulphur smells from throughout its territory overnight.

"While it is surprising that the sulphur has been smelt so strongly in the lower parts of our region, we don't consider it to be a cause for concern," emergency manager Shane Bayley said.

Council air quality monitoring sites in Taumarunui and Taihape were not showing any elevated presence of fine air particles. The public would be alerted if that changed.

WHAT TYPE OF ERUPTION

Rising magma sparking earthquake swarms and superheating Tongariro's geothermal systems in recent weeks is the most likely cause of Monday's steam-driven eruption.

GNS Science vulcanologist Nico Fournier said volcanic gas and periodic swarms of about 100 earthquakes had been recorded since scientists first noticed tremors on July 13.

Tongariro had experienced only one or two shakes a year in the past decade.

The earthquakes were centred two to seven kilometres beneath the mountain and had probably been triggered by magma "looking to find a way out".

Canterbury University hazard and disaster management lecturer Thomas Wilson said the volcano's hydrothermal system had probably exploded in a sequence known as a phreatic eruption.

"It's existing rock and material underneath the volcano that are fragmented and erupted. So in a purely steam-driven eruption, there's usually no new magma."

Two other types of eruption had yet to be ruled out - a phreatomagmatic eruption, where water and molten rock mixed, causing the magma to fragment in an explosive eruption; and a purely magmatic eruption, involving no water.

Wilson said the rumbling could either stop completely, continue with same-size eruptions, or it could be the start of a "bigger eruptive sequence, which would probably mean larger eruptions, with more ash produced, probably leading to more widespread deposition of ash, to a thicker amount".

The most likely cause for the eruption was rising molten rock 1km to 5km beneath ground level heating and destabilising the hydrothermal system.

Hydrothermal eruptions tended to recur and could escalate into larger, energetic molten eruptions which spewed out lava and rocks.

Lack of rain, the small size of the eruption and mild ash fall meant there was no lahar.

A HISTORY OF VOLCANIC ACTIVITY

Mt Tongariro has a well-documented history of volcanic activity, Rosenberg says.

Eruptions occurred in 1869, 1892, 1896 and 1897.

Newspaper articles from the 1800s describe previous eruptions in the area.

"The scene was continually changing, the most interesting outbursts being best seen from Taupo," a 1896 newspaper clipping from Hawera & Normanby Star said.

"One day it was Ketetahi, next it would be Te Mare [sic], while Ngauruhoe sent up a fairly regular quantity of white steam, the other two being erratic."

Another from 1897 describes a "state of desolation, strewn with large quantities of mud, stone and sand".

"The eruption had formed a new crater higher up the mountain, blowing up some of the overhanging cliffs.

"Rocks estimated at about 4 tons weight had been hurled up, ascending 600ft above the mouth of the crater and falling over half a mile distant. The old crater has been filled up with debris, but it continues to steam. Liquid mud has flowed into Lake Rotoaira," the Evening Post report said.

A Bay of Plenty Times story from 1892 said the eruption did not appear to damage the mountain, but had ejected "an immense quantity of stones, ashes, etc . . .

"The sight is described as being terribly grand."

There were 10 to 20 craters in the area that had been active in the past 10,000 years, Rosenberg said.