People in the Wellington region are being called on to delay all non-essential travel.
With roads greasy from rain, the slippery conditions have caused many minor crashes on main routes.
There have already been major delays for motorists, and traffic flows in the region are expected to increase markedly during the next few hours.
Inspector Steve Darroch has urged motorists to drive to the conditions. He predicts bottlenecks on State Highway 1 at Paremata and also between Pukerua Bay and Paekakariki.
North of Auckland, there are major delays for holidaymakers as they head into Wellsford.
Inspector Earl MacKintosh says northbound motorists need to allow extra travel time or use alternative routes.
He says motorways around Auckland are also heavily congested.