Gas levels in Mt Ruapehu's Crater Lake are the second highest ever recorded, creating concerns of increased volcanic activity.

The heating phase started last month, but elevated levels of unrest had continued over the last week and the prolonged period was not typical, a GeoNet alert issued on Tuesday said.

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

The high gas output, volcanic tremors, and slowing rising temperatures at Te Wai ā-moe, also known as Crater Lake, indicated the unrest was being caused by molten rock or magma.

An airborne gas monitoring flight on Monday showed gas output levels – particularly carbon dioxide – were the second highest ever recorded.

Tremor levels remained elevated, but have declined from the peak reached on April 6 and 7.

The temperature of Te Wai ā-moe was 38C – which was still in the typical range for a heating cycle.

'The most likely outcome of this unrest episode is no eruptive activity occurs, as no eruptions have followed unrest in the past 15 years, says duty volcanologist Brad Scott.

But, Mt Ruapehu is still an active volcano and had the potential to erupt with little or no warning when in a state of elevated volcanic unrest.

There is a possibility of one or more eruption that could impact the summit area and generate lahars – mudflows caused by lake water ejecting onto the ice and snow.

The size of these eruptions, if they did occur, would probably be like the eruption on September 25, 2007, says Scott.

The explosive eruption lasted around seven minutes and caused ash, rocks and water to be spread across the summit area.

The chances of a prolonged and larger eruption, like in 1995 to 1996 with wider ashfall, was possible, but very unlikely.

The most common hazards in a state of unrest at level two were steam discharge, volcanic gas, earthquakes, landslides, and hydrothermal activity.

GNS Science and its National Geohazards Monitoring Centre are continuing to closely monitor the volcano.

-Stuff.