The Department of Conservation is urging climbers and trampers to stay away from the summit of Mt Ruapehu after warnings the volcano could erupt.

A summit hazard zone - the area within 2km of the centre of Crater Lake - should be avoided, department Ruapehu area manager Jonathon Maxwell said on Friday afternoon.

The warning did not affect nearby Mt Tongariro and the popular Tongariro Alpine Crossing are not affected, but people should be aware of an increased possibility of lahars in streams on the Round the Ruapehu Mountain track.

On Friday morning, GNS Science said pressure was building under Mt Ruapehu and there was an increased chance it will erupt.

The aviation alert level around the central North Island has been raised to yellow - suggesting unrest above background levels - but the volcanic alert level remains unchanged at "signs of unrest".

GNS Science volcanologist Steven Sherburn says the changes make scientists believe eruptions are more likely in the coming weeks to months.

"We are monitoring Ruapehu closely, but it often does not give any immediate warning that it is going to erupt.

"We think that the temperature a few hundred metres beneath Crater Lake is about 800C, but the lake itself is only about 20C. This suggests the vent is partly blocked which may be leading to a pressure build-up beneath Crater Lake.

"A sudden release of the pressure may lead to an eruption."

Since late-October small earthquakes have recorded about 5km beneath Ruapehu's summit, but they may not be directly related to the high temperatures beneath Crater Lake.