Heavy rain has caused a lahar - a stream of ash and mud - from Tongariro's northern slopes to close a section of Highway 46 near Lake Rotoaira, but a GNS volcanologist says this is completely in keeping with the after-effects of eruptions.

Michael Rosenberg said a member of the public had reported about 13cm of mud crossing the road around 8.30am today.

"I guess the message is it's not a volcanic triggered lahar.

"It's the kind of thing we would expect when there's a lot of ash and debris. It's a very common thing that happens on a volcano.


"This is something we expect with high rain.

"People might have seen pictures of the awesome mudflows in Indonesia on their volcanoes after their heavy tropical rain washes away the ash. This is not something unusual."

Mr Rosenburg said the lahar was nothing like the dangerous bodies of water and sludge which have thundered off Mt Ruapehu in the past. In 1953 a fatal lahar swept away the Tangiwai Rail Bridge killing 151 people travelling on the Wellington to Auckland service. In 2007 a lahar caused the crater lake to burst.

A team of Massey University researchers are heading to the lahar.

Earthquake and other volcanic activity had been quiet overnight, Mr Rosenberg said.

Department of Conservation (DOC) spokesman Bhrent Guy said the lahar caused a "truck-load" of ash, debris and water to run down SH46.

"It's just a small blocked culvert on the side of the road, nothing substantial."

DOC workers are sweeping the road clear.


Rotoaira local Dave Bennett said it was not unusual for that part of the road to flood in heavy rain.

"I've been up there and had a look. It's a normal place where the road washes out and there's a little but of ash in it, that's about all."