Prime Minister John Key says he was made aware a few days ago that there was increased activity on Mt Ruapehu.
GNS Science revealed today the likelihood of the volcano erupting over the next few weeks had increased.
Duty volcanologist Steven Sherburn said changes in measurements over the last few weeks indicated eruptions are "more likely over the next weeks to months''.
"We're monitoring Ruapehu closely, but it often doesn't give any immediate warning that it's going to erupt,'' Dr Sherburn said.
"We think that the temperature a few hundred metres beneath Crater Lake is about 800 degrees Celsius, but the lake itself is only about 20 degrees Celsius.