"The most likely outcome of this unrest episode is no eruptive activity occurs, as no eruptions have followed unrest in the past 15 years," GNS duty volcanologist Brad Scott said.
"However, there is also a possibility of a single or multiple eruption that could impact the summit area and generate lahars into some catchments draining off the volcano, especially the Whangaehu Valley."
"The size of these eruptions, if they did occur, would probably be like the September 2007 event."
The September 2007 eruption lasted less than a minute but shot ash, mud and rocks up to 2km away.
One climber was hurt by a surge of water and debris running down the mountain.
Scott said the chances of a longer and larger eruption, such as occurred in 1995-96 with wider ash fall impacts, was possible, but remained "very unlikely".
"Such an eruption would most likely only follow a sequence of smaller eruptions," he said.
The series of eruptions in 1995-96 blew water out of the Crater Lake and created eruption columns up to 10km high with the 1996 eruption producing more than 7 million tonnes of ash that contaminated water supplies, destroyed crops and led to the death of livestock.
Scott said scientists had set the current Volcanic Alert Level to 2.
"Mt Ruapehu is an active volcano and has the potential to erupt with little or no warning when in a state of elevated volcanic unrest," he said.
"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."
Under level 2, the most expected hazards are steam discharge, volcanic gas, earthquakes, landslides and hydrothermal activity.
But eruptions can still occur with little warning.
GNS Science volcanologist Geoff Kilgour earlier this month told RNZ there's a lot of gas and fluid moving through the volcano.
"If you think about tremors as like very small earthquakes that occur quite frequently, like a balloon that is filled up and then released, it's like air moving through the outlet quite quickly.
"That's what we're seeing at the moment. We know that there's quite a lot of pressure in the volcano and so therefore the likelihood of an eruption has certainly increased."
Beneath the lake is a hydro-thermal system and there must be some sort of pressure gap holding a lot of the pressure back but still enabling some of it to come through, resulting in tremor, he said.
"The volcanic alert level settings that we have, we go from essentially from background level, minor unrest and then heightened unrest. As we go up those alert levels we generally expect the probability of an eruption to increase."
