Photo: GNS Science
Ruapehu crater lake / Te Wai ā-moe is at its lowest temperature in more than 50 years.
GNS Science said the volcanic alert remained at level one on its five-level scale, with an eruption unlikely - less than a 1 percent chance in the next one to three months.
The lake temperature was currently at 8C, the coldest it has been since 1970.
GNS Science said history showed there was a slightly higher than normal probability of an eruption during cool periods.
Volcanologist Steve Sherburn said when the temperature of the lake was both hotter and colder than normal it made it a bit more likely that there could be an eruption.
He said eruptions were more common during periods of warmer lake temperatures because molten rock entering the lake means more heat. And "more heat means more activity, which means more eruption".
A cooler lake and lower levels of volcanic gas emissions was indicative of partially blocked vents - the current situation at Ruapehu - which also slightly increased the chance it could erupt.
"What we are seeing here is a very different mechanism. The plumbing system beneath the volcano has some partial blockages, with minerals sealing the vents."
He said blocking the vents caused pressure to build, and this could trigger an eruption, which happened three times in Ruapehu's monitored history.
Ruapehu erupting in 1995. Photo: GNS Science / Lloyd Homer
He said the most recent, in 2007, was preceded by 10-15 minutes of volcanic activity, and since then GNS' monitoring system was now around-the-clock.
Sherburn said while an eruption remained "the $64 million question," the current modelling showed a chance of Ruapehu erupting in the next one to three months was less than 1 percent, or "comparable to the chance of getting hit by a car".
