"We are receiving a large number of calls related to this rain event."
"If your home or business is being impacted by surface flooding you can contact us on 06 349 0001, but please keep this line open for urgent calls only this evening."
There are multiple slips on State Highway 3 north of Rātana, where the cliff face has crumbled onto the road.
The road is currently reduced to one lane.
A police officer at the scene said contractors were on the way.
SH3 is also closed at Warrengate Rd south of Whanganui with a detour in place via Fordell.