The Department of Conservation says the risk for trampers on lower parts of the Tongariro Alpine Crossing has reduced after a dam broke, releasing volcanic debris.
The tourist attraction is scheduled to reopen fully on Friday for the first time since Mt Tongariro erupted in August this year.
Debris from the eruption blocked a stream tributary and created a dam.
DoC scientist Harry Keys says rain at the weekend caused the lake level to rise and the dam wall to break, resulting in a lahar flowing down the mountain and across State Highway 46.
Mr Keys says DoC had worked out how to manage the risk of a potential lahar from the dam, but the weekend's event means the hazard is now much reduced.