Snow has fallen to 400 metres in lower parts of the South Island and thunderstorms could be on the cards for the top half.
MetService duty forecaster Bill Singh says an unstable southerly flow is the culprit behind the snow.
"As we move towards the day, the snow level is expected to rise to about 600 metres," he said.
"There has been a warning out for central Otago, inland Clutha and Southland for snow ... to [fall] to 800 metres where between 30 to 40 centimetres of snow is expected to accumulate."
Mr Singh says there's also a chance of thunderstorms this afternoon.
"With the unstable air mass we've got a moderate chance of thunderstorms over inland areas of the upper South Island and a very small chance of severe thunderstorms over inland Marlborough areas this afternoon."
Thunderstorms are expected to bring localised heavy rain of 10 to 15mm per hour as well as hail.
Snow is expected to settle on roads around the South Island and snowfall warnings have been issued.
The worst effected Lindis Pass can expect 10-15cm of snow this morning, while 5-10cm is expected to settle on Crown Range Road, and 6-12cm on Milford Road.
Lewis Pass, Arthur's Pass and Porters Pass can also expect snow between 1cm and 6cm.