A new hazard map highlights the areas around Mt Tongariro most at risk from flying hot rocks or pyroclastic mudflows should the volcano erupt again.

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.

Scientists say there is still a 50/50 chance that Mount Tongariro could erupt - and it could be at any time.

Former TVNZ newsreader Judy Bailey says good timing probably saved her from near disaster.