Last month was NIWA's second-warmest July on record

South Island mountain towns such as Arthur's Pass and Mt Cook village are covered with snow, and thunder has rumbled across to the southern North Island with downpours across central areas.

However, some northern areas also saw bursts of sunshine today - temperatures reaching 20C in Whangarei and 18C in Auckland.

Heavy rain warnings from MetService cover North Otago, the Canterbury plains and foothills, and south of the Rangitata River below the 300m to 400m snow level through until mid-morning.

Above 300m to 400m - including in Dunedin - snow is forecast until mid-morning, with significant falls of 30cm to 40cm above 500m.

Dunedin, in areas below 300m, is under a heavy rain watch.

There is also a warning of heavy rain in the Tararua Range this evening, with thunderstorms possible.

Northern areas including Waikato, Waitomo, Taumarunui, Taranaki, the ranges east of Whakatane, as well as Buller and west of Motueka and the Canterbury plains and foothills, may also experience thunderstorms.