The Department of Conservation is warning people to stay away from Mount Ruapehu's summit because of concerns it could erupt.
DoC says people should stay outside the Summit Hazard Zone, 2km from the Volcano's crater lake.
GNS Science says recent tests reveal a build-up of pressure under the crater lake which, if suddenly released, could cause an eruption without warning.
Ruapehu erupted without warning in 2007.
A conservation analyst for DoC, Harry Keys, says because of the volcano's unpredictable nature they have to take the safest option and recommend people stay away from the summit.
"Ruapehu's very variable, it won't stay in a situation for long. What does long mean ... weeks, maybe months but weeks more likely.
"Eventually some event will happen to unblock it, whether that event is a small one like we had in, say, July 2009, or a large one that happened in September 2007 or something else, that's still impossible to tell at this stage."
He says anywhere outside the 2km summit hazard zone is safe for climbers and trampers.
GNS Science says a vent to the crater lake seems to be partly blocked, meaning a sudden release could lead to an eruption.
Volcanologist Steven Sherburn says the volcano is being monitored closely, but it does not often give immediate warnings of an eruption.
He says the crater lake is measuring only 20 deg C at the moment, but the temperature a few hundred metres below the lake's surface is 800 deg C.
Mr Sherburn says there's probably some sort of blockage, meaning the heat cannot escape to the top of the lake.
He says if the heat can release slowly, the lake will just get warmer and no eruption will occur.