A significant amount of rain battered Auckland on Monday and motorists are being urged to reduce speed and increase following distances.
It comes after a severe thunderstorm watch was issued for parts of the North Island, including Northland, Auckland, Waikato, Waitomo, Taumarunui, western Taupō, Rotorua, Bay of Plenty and near the Kaimai Range.
MetService said scattered thunderstorms are expected to bring heavy rain and hail, with the potential for severe thunderstorms in some areas between 2pm and 9pm on Monday.
The downpours have flooded roads and major motorways in and around Auckland.
Insp Jason Homan urged motorists "to take extra care to avoid any preventable accidents".
"Following too close to the person in Infront of you or attempting to overtake in risky situations is a recipe for disaster," he said.
"Reduce speed as it is a contributing factor in a third of fatal crashes and is involved in 71 percent of injuries. This only increases in bad weather conditions."
Flooding has forced the closure of the left lanes on State Highway 1 at Mt Wellington after Penrose Rd.
"Please expect delays and pass with care," Waka Kotahi said.
Homan said contractors are working to clear flooding on the motorway network.
"People are asked to take extra care when passing roading crews."
MetService said the storms could bring localised downpours of 25-45 mm/h and a low risk of a small, localised tornado.
The rest of the country has generally fine weather on Boxing Day, the forecaster said.
