Snow and ice closed several main arterial routes in the central North Island today, causing headaches for holidaymakers at the start of the long weekend.

The Desert Road, SH1 between Rangipo and Waiouru, reopened at 12.30pm but extreme care was required.

Earlier reports had suggested diversions would stay in place for the rest of the day, but better weather conditions meant traffic could resume with caution this afternoon.

State Highway 49 between Waiouru and Ohakune, State Highway 4 between Wanganui and Taumarunui, and State Highway 47 between National Park and Junction SH41 and SH47 were re-opened by 10am.

A landslip in the Manawatu Gorge was causing traffic delays between Woodville and Palmerston North.

Police said State Highway 3 through the gorge remained "narrow" in places, speed restrictions of 50km or 70km were in place and "extreme care" was required.

In the event of rain, motorists were advised the road could be closed at short notice.

A spate of early morning accidents further snarled up traffic.

A truck and trailer unit became disconnected and flipped around 2.30am, completely blocking the south-bound lane of State Highway 3 near Urenui, 32km northeast of New Plymouth.

Meanwhile, an accident on the Napier-Taupo road early this morning also closed the southbound lane.

Inspector Kristy Meates from the police central communications centre said it appeared a car lost control in the icy conditions and "slid into a bridge".

No one was reported injured in the accident but substantial delays were expected.

"There's a little bit of snow and we're urging drivers to exercise extreme care," she told NZPA.

All South Island roads were open this morning, although the AA was urging "extreme caution" where heavy snow fell yesterday and overnight.

Chains were essential and towing was forbidden on mountain passes.

Yesterday's major snowfall in Dunedin - the first of the year - was a windfall for towing operators, as hundreds of motorists were forced to abandon their cars.