A weather bomb is expected to lash parts of the country tomorrow bringing gales of up to 160km/h, torrential rain and a chance of snow on higher ground.
Damaging winds were forecast for parts of the South Island and lower North Island from the morning, while the West Coast is in for extremely heavy rain later in the day.
Westland is expected to have the most rainfall, but heavy downpours are expected for Buller and Fiordland and severe winds could hit Canterbury, north of Ashburton, Marlborough, Wellington and the Wairarapa.
Gusts of 150-160km/h were expected in exposed places from late Wednesday afternoon. MetService warned that "winds of this strength could bring down trees and powerlines, damage unsecured structures, lift roofs and make travel hazardous".
A low pressure system that deepens very rapidly in a short time was known as a "bomb low", MetService said.
MetService meteorologist Lisa Murray said it would bring extremely large amounts of rain - up to 300mm in 24 hours - for some of the South Island's ranges. This could cause widespread flooding, slips and damage to roads.
"Rivers and streams will rise rapidly, including those in Canterbury that feed from the Alps.
"This event will especially impact anyone in the mountains, river users, tourists and those travelling on the roads."
As the low moves off the country to the east on Thursday it will bring unseasonably cold overnight temperatures, with some areas dropping to 4°C.
Ms Murray said the cold temperatures could even bring snow overnight Wednesday through to Thursday.
"Can you believe at this time of year, there is a chance that Eastern Otago and parts of Fiordland could get some snow above about 700m?".
The storm is likely to bypass Auckland and the upper North Island where summer highs are set to continue.
Fire risk for Canterbury
The strong winds are expected to create an extreme fire risk in Canterbury tomorrow, with residents being asked to take precautions.
Selwyn District principal rural fire officer Douglas Marshall said people should avoid activity that could start a blaze, including operating machinery, in hot weather.
Power outages could also occur and people should be prepared, he said.