Volcanic tremor levels at Mt Ruapehu increased over the past week and are now among the highest seen in nine years.

The Crater Lake (Te Wai ā-moe) temperature has continued to rise, and GeoNet says the lake was at 36C on Tuesday.

It remains in a state of unrest at alert level two - a higher risk of an eruption.

GNS science volcanologist Geoff Kilgour told Morning Report this tells them there's a lot of gas and fluid moving through the volcano.

If think about tremor 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 says.

"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."

Generally speaking, volcanoes go through periods of unrest far more than periods of eruption, he said.

"The chances of an eruption are still quite low and far more likely that it's not going to result in an eruption, but it's clear that there is pressure building up immediately beneath the volcano and therefore the likelihood has certainly increased from before."

Scientists will be doing a lot more data gathering expeditions to reduce uncertainty and to provide as much information as possible, he says.

-RNZ,