A section of State Highway 1 in Northland has been closed because of an underslip on the southern side of the Brynderwyn Hills.
The highway was closed just after 1pm on Sunday when the northbound lane collapsed after heavy rain.
Transport Agency spokesperson Jacqui Hori-Hoult said the agency would have experts on site on Sunday afternoon assessing the stability of the hillside, and it was not known yet when it would be safe to reopen the highway.
A detour is available via State Highway 12 through Paparoa and Oakleigh.
The Fire Service said three homes in Maungaturoto, south of Whangarei, were flooded; in one case, the occupants of a house were trapped by flood water.
Crews also helped three stranded vehicles, at Maungaturoto, Waipu and Tutukaka.
Civil Defence says the worst affected areas are around Whangarei, the Bynderwyns and inland towards Dargaville.