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Hailstones the size of golf balls battered motorists between Tauranga and Taupo yesterday evening, forcing police to slow traffic for almost an hour.
The hailstorm was the peak of a "potentially deadly" burst of weather over the upper North Island, which was expected to pass by sunset.
Inspector Jacqueline Whittaker said police patrols were set up either side of the Kaimai Ranges during a hail storm to ensure motorists slowed down and drove carefully. "We had reports of hailstones the size of golf balls, which would make driving very dangerous."
She said it was very rare for police to slow down traffic because of hail. "It is not common practice."
Driver Bev Adair was caught in the storm heading south on the Kaimai ranges. "It was incredibly scary. All of sudden it was just white, like I was driving on the top of Mount Ruapehu. It sounded like rocks hitting my car.
"Cars and motorbikes lined the banks of the road, because people who tried to stop just slid off the road."
The hail storm had completely subsided by 7pm, after half an hour, but weather remained torrid throughout the region.
Weatherwatch released a warning at 6pm for people near Tokoroa, Tauranga and Matamata to stay inside, because frenetic lightning strikes made outdoor activity dangerous.
Weatherwatch head analyst Philip Duncan said: "Thunderstorms like this can be deadly ... this is no exaggeration. Last year, an hour after we released a lightning warning, a man and his horse were killed by a lightning strike."
Mr Duncan observed more than 800 flashes of lightning between 6.45pm and 7.45pm, mostly between the Kaimai ranges and Bay of Plenty.
He said the extreme weather was caused by high humidity and unstable air pressure over the upper North Island.