Temperatures in some areas will get as cold as -9 overnight, says MetService.
The Twizel and Mount Cook area will be almost double digits in the negative.
Showers are expected in the east of the South Island, with snow to 400 metres in the Kaikoura ranges.
The North Island will see widespread showers, and snow to 600 metres.
Waiouru is forecast to be the coldest spot in the North Island, at -2.
Showers are expected to come down hard in Taranaki and Taumarunui in the morning.
The Desert Road also has a snowfall warning in place, along with State Highway Five between Napier and Taupo until 3 am Monday morning.
MetService weather warnings on 16 August. MetService/Screenshot
Road snowfall warnings were in place for Napier-Taupo Road (SH5), Desert Road (SH1), Lewis Pass (SH7), Arthur's Pass (SH73) and Porters Pass (SH73) from Sunday morning into the evening and Monday morning.
All state highways in the South Island are open this morning, despite snow sweeping through overnight.
However, the Transport Agency says motorists should take care on the roads where snow has settled.
-RNZ