Taupo District Council is investigating reports of a lahar streaming from Mount Tongariro.
The council says it is assisting GNS Science with investigations and heavy rain is expected to push debris from Monday night's eruption down the mountain.
Head of the volcanology department at GNS Science, Jill Jolly, says State Highway 46 has been closed as a precaution.
Jolly says it might be too early to call it a lahar, so council staff will make observations and report back.
She said this is something which would be expected after an eruption, but it is not directly related to increased volcanic activity on the mountain.
There are reports of heavy surface flooding in the Rotorua and Taupo areas after heavy rain.
GNS says the activity from Tongariro Volcano remained weak overnight.
Seismicity in the local Tongariro area has remained low. The "noisy" signal on the seismograph drums is due to windy weather conditions, said duty volcanologist Michael Rosenberg.
The Volcanic Alert Level remains at level 2.