Mt Ruapehu has had its longest period of shaking in the past 20 years as elevated volcanic unrest continues.
This heightened unrest - which has been going on for four weeks - is mainly volcanic tremors and heat flow into the summit crater lake, GeoNet volcanologist Agnes Mazot said.
The crater lake temperature has stabilised at 37C for the past week and the volcanic alert level remains at two.
"The tremor levels remain elevated, representing the longest period of tremor recorded over the past 20 years," Mazot said.
"An observation flight and sampling of the crater lake were performed last week to further assess the activity. Our scientists observed some upwelling in the lake, indicating that hot gas and liquid are still making their way into the lake."
The lake's temperature has increased by 7C over the past 30 days, although this is similar to other heating phases, Mazot said.
Despite the heating stabilising at 37C, the ongoing volcanic tremors indicate heightened unrest since pressure could be building up at depth.
"The sustained nature of the volcanic unrest, coupled with the volcanic tremor signals, elevated gas output and hot lake remain indicative of processes being driven by molten rock (magma) interacting with the geothermal system within the volcano," Mazot said.
The order of the potential outcomes hasn't changed since last week, she added. The most likely outcome of this unrest episode within the next four weeks is still that no eruption activity happens since no eruptions have followed unrest in the past 15 years.
"However there is also a possibility of a single or multiple eruptions that could impact the summit area and generate lahars into some catchments draining off the volcano, especially the Whangaehu Valley," Mazot said.
"The size of these eruptions, if they did occur, would probably be like that of September 2007, or the slightly larger April 1975 or June 1969 events.
"The chances of a prolonged and 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."
Volcanic alert level two indicates the primary hazards are those expected during volcanic unrest, which are steam discharge, volcanic gas, earthquakes, landslides, and hydrothermal activity. While level two is mostly associated with volcanic unrest hazards, eruptions can still occur with little or no warning.
