Rail company TranzMetro is to investigate long delays for commuters in the Wellington region caused by ice on overhead cables.
About 7000 passengers, mainly on the Kapiti line, had a cold, slow journey to work as a buildup of ice on wires at Kenepuru and Tawa halted or slowed trains.
TranzMetro Wellington general manager David Shepherd said several early morning trains tried to get through but got stuck, and once they got going again, could only travel along the lines very slowly.
He said having iced-up cables on the network was rare but the impact it had this morning warranted an investigation.
The icy conditions also caused several crashes in the Wellington region this morning.
Police said one woman was taken to hospital with chest injuries after her car slid into a ditch on State Highway 58 near Moonshine Road. There was also another crash on State Highway 58 and one in Aro Valley in central Wellington
Temperatures in many parts of the country plummeted overnight, with a low in Timaru of -7°C.
Also in the South Island, Christchurch was a little over 1°C at 8am, and Dunedin had warmed up to nearly 3°C after recording -6°C at the city's airport.