MetService is warning that temperatures could plunge to zero in parts of the country over the next two nights.
Severe weather warnings have been in place in Hawke's Bay and eastern Malborough as heavy rain and southerlies sweep the country, with Tararua and Wairarapa experiencing the heaviest rainfalls.
MetService said Hawke's Bay could be in line for a drenching tonight. The area north of Tutira in particular was expected to receive between 75mm and 100mm of rain, with the possibility of localised downpours of 35mm an hour.
The rain is expected to clear by tomorrow morning, with a low pressure trough moving away and making way for a "vigorous and cold" southerly flow.
Metservice said the combination of clearing skies and easing southerlies would bring a nationwide chill.
Meteorologist Stephen Glassy said places south of Auckland would experience temperatures in the single digits.
"Conditions are going to improve across most of the country, however the main thing to look out for is that it is going to be unseasonably cold for this time of year.
"Especially the overnight minimums - Tuesday morning is looking particularly cold for this time of year over most of the country."
MetService forecast Waiouru as the coldest centre on Tuesday morning, predicting a 1C minimum temperature.
Several other centres on Tuesday are set for temperatures around 5C and 6C.