A cold southerly blast is headed for the lower South Island tonight.

Snow showers are expected to lower to 100 or 200 metres in parts of Southland and Otago.

Strong winds will mean a sub-zero windchill in exposed places.

Earlier, Metservice warned of gale force winds in the Wairarapa region.

Gusts of up 120 kilometres an hour were possible, strong enough to bring down trees and power lines.

In the upper North Island, isolated thunderstorms and hail are forecast tonight.

Tomorrow's weather will continue to be affected by a cold southwest flow, with snow showers predicted on the Desert Road in the central North Island. There will be snow on the higher parts of the road from late morning, lowering to 700 metres in the afternoon.

In the Wellington area, snow showers are expected near the summit on the Rimutaka Hill Road tomorrow afternoon.

Snow showers are also expected on several South Island roads and passes tomorrow.