Scientists are keeping a watchful eye on the crater lake on Mt Ruapehu, following a drop in its temperature.

Volcanologist Dr Harry Keys said GNS Science and the Department of Conservation are monitoring the volcano as there is a small chance of a small eruption.

"When the magmatic heat slows down, the lake cools off. That's the normal thing, and everything is pointing to that, but there is just a little bit of uncertainty that it could be due to a sealing or a partial blockage in the vent system below the lake."

People are being warned not to go within 400 metres of the crater lake.

He said it's business as usual for the rest of the peak, and the warning is aimed at the few climbers and skiers who head up to the crater.

"We're suggesting people don't camp up in the summit area, don't linger up there. And then don't linger in the bottom of the valleys in the Whangaehu or the Whakapapaiti, where sometimes lahars can go."