More rain tomorrow night was also possible.
MetService meteorologist Angus Hines said people in the Bay of Plenty and Coromandel should be prepared for surface flooding and poor visibility as the rain ramps up early tomorrow.
A low-pressure system to the North will move a belt of heavy rain into the eastern part of the North Island, he said.
He said slips were possible.
Waka Kotahi NZ Transport Agency regional manager maintenance and operations Rob Campbell said to "expect the unexpected" and urged drivers to take care and avoid unnecessary travel.
The heavy rain can cause streams and rivers to rise quickly, and the agency may have to close some roads, he said.
"Check your intended route for hazards or disruptions before setting out using the Waka Kotahi interactive Journey Planner website (journeys.nzta.govt.nz) and be prepared for unreported hazards."
Campbell says on a wet road, driving at the speed limit could be too fast as it's easy to lose control, especially if needing to brake suddenly.
He said it was also important to increase following distances.
He asked motorists to be mindful of contractors who could be out clearing hazards off the road, and drive slowly through work sites.
"We want to see everyone get home safely. Stay safe and stay alert."
