Mount Ruapehu's crater lake is showing signs of heating up, but the alert level remains low.
GNS science volcanologist Brad Scott said the lake's temperature had risen from 15 degrees celsius in early December to 41 degrees in the past few days.
Mr Scott said the lake's colour indicated that volcanic gas was coming up through the lake bed.
"What that does is it disturbs the sediments on the floor of the lake and it will change from a blue-green colour to a grey colour and currently the lake is a grey colour."
Mr Scott said the activity alert level remained at one-out-of-five.
He said the lake appeared to be at the hottest point of its nine-to-14 month cycle and it was now expected to begin cooling.
The lake last erupted in 2007, when two climbers were injured, but there was no warning.