The mercury reached 26degC at 4am at Dunedin Airport this morning, while Alexandra had reached 24degC and Invercargill 22degC by 7am, according to MetService data.
MetService meteorologist Claire O'Connor told RNZ the warmer temperatures were expected towards the start of autumn, and predicted a hot day in the east of the South Island.
However, by mid-afternoon she said there was potential for a "pretty sudden drop", as the winds changed.
Wind watches were also in place for the Canterbury High Country, Canterbury Plains and inland Otago.
"Northwest winds may approach severe gale in exposed places," MetService said of the Canterbury High Country, with potential damage to trees and powerlines.
"Prepare your property by securing items that can be picked up by strong winds."
But Cantabrians could also expect a bit of sun too, O'Connor said, with temperatures up to 30degC.
Meanwhile heavy rain warnings are in place for Westland and Fiordland.Westland residents could expect up to 200mm of rain around the ranges and 100mm near the coast between 10am Monday and 6am Tuesday, MetService said.
Peak rates will reach up to 40mm about the ranges. There was a minimal chance this 'orange' warning could be upgraded to 'red', meaning a "most extreme weather event".
"Streams and rivers may rise rapidly. Surface flooding, slips, and difficult driving conditions possible," MetService said on its website. "Clear your drains and gutters to prepare for heavy rain. Avoid low-lying areas and drive cautiously."
O'Connor said heavy rain like this had been rare in the region in recent months.
"This is a heavy rain warning that they haven't really seen in these areas for basically most of the year… This is one that's going to be, probably a bit impactful given that most places along the West Coast are sitting well below their rain for the year to date. So it's a lot more rain than what they've seen for the last three or four months."
In Fiordland, up to 160mm was expected between 7am and 10pm Monday, with a peak rate up to 30mm an hour "especially in thunderstorms". There was a minimal chance of the orange warning upgrading to red.
Less urgent heavy rain watches were in place for Southland (3pm Monday - 3am Tuesday), mainly in the west, with a high chance of upgrading to a warning; and Stewart Island, for 12 hours from 9am Monday, with a low chance of upgrading.
O'Connor said the wet weather might be good news for the region's hydro lakes, which were low for the time of year.
Meanwhile in the North Island, forecasters are predicting strong winds in the capital. From midnight Tuesday morning through to noon, north to northwest winds "may approach severe gale in exposed places". There was a moderate chance this watch could be upgraded to a warning.
- APL and RNZ
