5pm update: Geonet Duty Volcanologist Geoff Kilgour says that the lake level is marginally higher, and the central vent seems to be actively upswelling.

"No seismic or acoustic activity accompanied the plume appearance, indicating the steam plume was not generated by strong activity in the lake," says Geoff.

"We cannot rule out that very minor activity has occurred, which is why our scientists conducted an observation flight early this afternoon.

GeoNet image of Ruapehu from this morning. Supplied Photo.

"Observations from that visit indicate that central vent continues to be actively upwelling, and the lake level is marginally higher. These observations are like those reported yesterday."

"The current lake temperature is now around 39 ºC. In the past, lake temperatures this high, combined with cool and calm atmospheric conditions, have generated visible steam plumes above the lake.

"Laboratory analysis of the latest Crater Lake fluid samples are now processed. Results are very similar to past analyses and show no clear indication that magma is interacting with the hydrothermal system beneath the lake.

"We will conduct further gas measurement and observation flights as weather conditions allow."

In the past three days, the temperature of the crater lake - Te Wai ā-moe - has risen to 38 degrees Celsius. Photo: Cindy Baker.

2pm update: Ruapehu locals are reporting that a white plume of volcanic gas and smoke can be seen coming from the summit of Mount Ruapehu following recent tremors.

A photo from Cindy Baker of Ruapehu Scenic Tours on Tuesday morning shows a plume ascending from Mount Ruapehu, as GNS Science also report that a period of heightened unrest continues for the volcano, with continued high levels of volcanic gas emissions and strong tremors.

The Mount Ruapehu crater lake temperature has risen to 38C.

Recent airborne gas measurements indicated the volcano had maintained high levels of volcanic gas emissions and strong volcanic tremors.

The Volcanic Alert Level of Mount Ruapehu remains at 2 and The Aviation Colour Code at Yellow.

The GNS Ruapehu earthquake map shows the number of earthquakes that have occurred over the last few weeks.

The Ruapehu earthquake map on the Geonet website. Image: GNS Science.

At 2797 m, Ruapehu is the highest mountain in the North Island. Three summit craters have been active during the last 10,000 years including South Crater which contains the currently active vent. The active vent is filled by Crater Lake; water from this lake is frequently ejected on to the ice and snow during eruptions causing lahars.

The most recent eruption was on September 25 2007. This was an explosive eruption, lasting about seven minutes. During the eruption explosions spread ash, rocks and water across the summit area, producing lahars in two valleys including one in the Whakapapa ski field. In contrast with the previous eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area.

Earlier, 9am: GNS Science is reporting that a period of heightened unrest continues for Mount Ruapehu, with continued high levels of volcanic gas emissions and strong tremor.

Now in its sixth week of raised volcanic activity, the past three days has seen the temperature of the crater lake -Te Wai ā-moe - rise to 38 degrees Celsius, following a four-week period at 36-37 degrees.

The Volcanic Alert Level remains at Level 2.

GNS Science duty volcanologist Geoff Kilgour says Mt Ruapehu has "exhibited the strongest volcanic tremor in two decades along with a rise in Crater Lake (Te Wai ā-moe) temperature".

In the past three days, the temperature of the crater lake - Te Wai ā-moe - has risen to 38 degrees Celsius. Photo: DOC.

"Over the last week, the level of volcanic tremor has varied, with bursts of strong tremor interspersed by short, periods of weaker tremor. This represents a change in character in the tremor, and the driving processes remain unclear."

"Our modelling suggests that to maintain the lake temperature and subtle rise requires ~200-300 MW," says Geoff.

Due to the heightened volcanic unrest, GNS Science staff are carrying out more frequent aerial gas measurements and Crater Lake sampling. A gas measurement flight on April 28 recorded the sixth highest sulphur dioxide (SO2) flux of 390 tonnes per day since 2003.

"Sulphur dioxide is a strong indicator gas and is derived from a relatively shallow magma body, which is perceived to currently exist a few kilometres beneath Crater Lake."

Geoff says further gas flights will be conducted when weather conditions are suitable.

"Sampling of Crater Lake was also conducted last week and during that visit, our scientists observed upwelling of Central Vent and reduced upwelling at the Northern vents area," says Geoff.

"During recent visits, active upwelling has only been observed at the Northern vents. It is important to note that Central Vent is the primary vent, whereas the Northern vents are a subsidiary vent system.

"We had surmised previously that Central Vent was sealed, blocking the main flow of fluids and gases into Crater Lake, however this vent now appears to be at least partially open," says Geoff.

"We are still awaiting laboratory analysis of the latest Crater Lake fluid and gas samples. This will show us if magma is interacting with the hydrothermal system beneath the lake. We will report on those when available."

The high sulphur dioxide (SO2) flux, sustained carbon dioxide (CO2) and continuing strong volcanic tremor, continue to indicate that molten rock (magma) is driving this period of heightened unrest. Increasing Crater Lake temperature is reflecting the increasing upwelling of hot fluids and gases through Central Vent as well as the Northern vents.

"Within the next four weeks, the most likely outcome of this unrest episode is no eruption, or a minor eruption that is confined to the lake basin," says Geoff.

"Small eruptions are still able to generate lahars, especially in the Whangaehu River. The next most likely scenario is an eruption that impacts the summit plateau and generates lahars in multiple catchments, similar to what was seen after the September 2007 eruption or older events like those in 1975 and 1969.

"The chance of a prolonged eruptive episode or a larger eruption, such as occurred in 1995-96 with wider ashfall impacts, is higher than it was two months ago, but remains very unlikely. Such an eruption would most likely only follow a sequence of smaller eruptions."

The interpretation of this activity is consistent with elevated volcanic unrest at the heightened level and therefore the Volcanic Alert Level remains at Level 2. The Aviation Colour Code remains at Yellow.

Mount Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of elevated volcanic unrest.

The Volcanic Alert Level reflects the current level of elevated volcanic unrest. The Volcanic Alert Level should not be used to forecast future activity. However, at Volcanic Alert Level 2, eruptions are usually more likely than at Volcanic Alert Level 1.

Volcanic Alert Level 2 indicates the primary hazards are those expected during volcanic unrest; steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity. While Volcano Alert Level 2 is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning.

For information on access to the Mount Ruapehu area, please visit the Department of Conservation's websites on volcanic risk in Tongariro National Park and follow the DOC Tongariro Facebook page for further updates.

For information about responding to volcanic activity there are guidelines from the National Emergency Management Agency's Get Ready website.

GNS Science and its National Geohazards Monitoring Centre continue to closely monitor Mount Ruapehu for further changes.