Photo: 123RF
Heavy rain warnings are in place for Westland and Fiordland, while Central Otago and Canterbury are in for a hot day.
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."
MetService meteorologist Claire O'Connor told Morning Report 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."
If 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.
Also in the South Island, wind watches were 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 30C.
"We will definitely expect sort of those warmer temperatures towards the start of autumn, but yeah, it's definitely a hot day through the east of the South Island.
"At Dunedin Airport this morning at 4am they were 26C. It was cooler in the city itself, you know - they're a bit separate. But, Alexandra is 24 at the moment and Invercargill 22C.
"So as those northwesterly winds arrive ahead of the rain, temperatures will pick up there."
Three flights between Dunedin and Christchurch were cancelled due to adverse weather.
An Air New Zealand spokesperson said the flights were cancelled due to high winds, low visibility and rain.
By mid-afternoon there was potential for a "pretty sudden drop" as the winds changed, she said. But until then, the hot, dry and windy conditions posed an elevated risk of fires.
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.
Wellington drivers are being advised to take extra care on the roads as strong winds look set to hit the region early tomorrow.
Waka Kotahi said people should take extra care on exposed roads such as Remutaka Hill on State Highway 2 and the Wainui Saddle on State Highway 1.
It said vehicles such as trucks, vans, motorcycles and towing vehicles in particular need to take extra care, but that all road users need to exercise caution.
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