Volcanologists say an eruption from Mt Tongariro could send clouds of burning hot ash onto State Highway 46 as well as homes on the shore of Lake Rotoaira.
GNS Science has issued a map showing flows of ash, lava and mud could travel up to 8km from the vents where Monday night's eruption happened.
The area includes parts of State Highway 46 and the Transport Agency is warning drivers not to stop on the road if there is an eruption.
However, GNS Science says another eruption is unlikely and would be preceded by an increase in activity from the mountain, giving time for warnings to be issued.
