Watch being kept on Tongariro lake

Debris damming a small lake created by Mount Tongariro's eruption could give way and send a cascade of water and mud across State Highway 46 and as far as Lake Rotoaira.

Conservation Department staff are monitoring the debris dam, in a situation similar to that on Mount Ruapehu when they waited for its crater lake to spill down the Whangaehu River in a lahar.

Only this situation was on a much smaller scale, DoC manager Nic Peet said.

The lake in question is only the size of a few big swimming pools and no one is living in its path.

The bridges it would wash out on the Tongariro Crossing are in the closed part of the track.

The only other precaution taken is "No Stopping" signs on either side of a culvert that runs under SH46, the road that runs east-west across the northern end of Tongariro National Park.

"It's keeping an eye on it rather than anything else at the moment," Dr Peet said.

A debris flow from the August 6 eruption blocked streams and created three small new lakes on the mountainside. The other two were now draining freely.

Dr Peet was the manager of both the Whanganui and Tongariro DoC areas at the time of the eruption. He said it took him by surprise at 11.50pm on that Monday night. Authorities were first alerted by Ngati Hikairo people, who saw ash falling and heard noise.

The mountain was quiet now, but the eruption opened a series of new vents. Steam and hydrogen sulphide - which smells like rotten eggs - was still being emitted. It can be smelt as far away as Wanganui at times.

The area of Mount Tongariro within two or three kilometres of the active vents was out of bounds for safety reasons.

If DoC staff had to be there they were there briefly.

The Tongariro Crossing track has been reinstated as far as Red Crater. The next piece to be opened would go as far as Emerald Lakes and allow through traffic from Oturere Hut.

The goal was make the whole crossing usable by Labour Weekend, Dr Peet said, and tourist operators were pushing for that. It would take another year to return steps and boardwalks to their pre-eruption condition.

Volcanologists say there are three possibilities for the mountain in the near future. It was most likely to stay quiet, slightly less likely to have another eruption of the same size and even less likely to have a bigger eruption. The longer it was quiet, the less likely it was to have a big eruption.