A slip north of Wellington has blocked State Highway 1.
Police say the slip is covering both north and southbound lanes on the stretch of road just north of Pukerua Bay.
Motorists have been told to expect "significant delays".
Waka Kotahi NZ Transport Agency (NZTA) says congestion is heavy between Pukerua Bay and the Poplar Ave onramp for southbound traffic.
"Due to a slip, the road is now closed in both directions at Pukerua Bay. Contractors are on-site.
"Please avoid this area & delay your journey. Please be mindful that Paekakariki Hill Rd is also closed due to fallen trees," NZTA tweeted.
A video posted to Facebook earlier on Thursday showed flooding in Lower Hutt, although a cafe in the area has since said the water has subsided.
Manawatū town Feilding is also bracing for more flooding, despite river levels dropping in the area.
Surface flooding hit the town on Monday after it received almost as much rain in a matter of hours - 78mm - as it normally would in a month.
Floodgates were closed to prevent a worse outcome, with rivers and streams completely full after a week of bad weather. Another 16mm had fallen by midday on Thursday.
"We will be keeping a close eye on the situation this morning as to date the weather has been somewhat unpredictable and changing very quickly," said Horizons Regional Council incident controller Craig Grant.
"The key rivers that we're watching are the Makino, Manawatū, Mangaone and Matarawa. Overall, our major river systems are coping as we'd expect however, all our catchments are saturated and it won’t take much to put pressure on them."
Earlier on Thursday, there was also flooding in and around Christchurch. Several streets in the city remain closed however, after some parts of the Canterbury region recorded rainfall in the triple-figures on Wednesday and the Avon River burst its banks.
