Increased volcanic activity at Mt Ruapehu has prompted a warning to climbers from the Conservation Department - though scientists can't say whether an eruption is coming.

The department said in a statement that there "may be an increased chance of eruptions" from Crater Lake and the risk to visitors approaching within 2km from the lake was higher than normal. Gas concentrations near the lake were high enough that they would affect some people.

GNS scientists had been monitoring activity levels at Mt Ruapehu since last September's eruption. Gas output and internal temperature levels had risen to levels outside the norm, but it was not known whether that was a sign of a coming eruption.

"If there is are further eruptions they may occur without warning," the statement said.

- © Fairfax NZ News