GNS Science says the volcanic alert level at Ruapehu remains at 1, despite the temperature of the crater lake rising towards conditions typical for an eruption.
Skiers on the Whakapapa ski field in the central North Island have reported smelling hydrogen sulphide gas in recent weeks.
Volcanologist Brad Scott says this is because the temperature of the crater lake is cooler, which tends to produce more gas.
The latest Volcanic Alert Bulletin says the lake's temperature on 29 September was 17.6 degrees Celsius. In March, the temperature peaked at 41 degrees Celsius.
Mr Scott says the lake is heading towards a temperature where volcanic activity typically occurs, but doesn't always. Ruapehu is an active volcano and future eruptions may occur with little or no warning.
The eruption detection system on the mountain functioned well during a test last week, he says.