The volcanic alert level at Mt Ruapehu has been raised to two. It was previously at one.
In a statement, duty volcanologist Craig Miller said another heating phase had started at Mt Ruapehu and Crater Lake (Te Wai ā-moe), with the temperature now reaching 31C.
It was currently overflowing into Whangaehu river.
"Typically, over periods of months, Ruapehu's crater lake undergoes heating and cooling cycles. A new heating cycle has started and has been accompanied by strong levels of volcanic tremor (volcanic earthquakes)," he said.