MetService says 15 to 25cm of snow is expected at settle on State Highway 1 at Leith Saddle from 1am tomorrow with lesser amounts down to 200m.
The forecaster updated its weather advisories late this afternoon to include an orange heavy snow warning for the hills of eastern Otago, including Dunedin, from 10pm today until 10pm tomorrow.
MetService said those areas could expect 25 to 35cm of snow to settle above 400m, with lesser amounts down to 200m.
Travel disruption and damage to trees and powerlines were possible and cold conditions may cause stress for livestock.
"If you must travel, drive cautiously, and ensure you have snow chains, sleeping bags, warm clothing, and emergency items," MetService said.
The wild weather has started affecting travel in the South. NZ Transport Agency Waka Kotahi advised at 4.40pm that a temporary speed limit of 30kmh was in place in the Cromwell Gorge (SH8) after a rockfall this afternoon.
About 2pm on Monday MetService advised that snow was settling on the Crown Range Road, while about 4.10pm the Queenstown Lakes District Council said chains were required to be carried by anyone travelling over it.About 4.40pm the Central Otago District Council said the Danseys Pass road was closed owing to ice and snow on the Waitaki side.
MetService earlier warned road users to expect snow on the Milford Road (SH94) and the Lindis Pass (SH8).
In an update late this evening, MetService also said an orange heavy rain warning was in place for North Otago, Dunedin and Clutha (southeast of Raes Junction) from 8pm today until 3pm tomorrow.
From 35mm to 55mm with peak rates of up to 10mm per hour was expected, on top of what had already fallen, and that rain was likely to turn to snow above 200m (possibly lower in some places) tomorrow.
"It’s going to be affecting quite a large area of eastern Otago and Dunedin," MetService meteorologist Mickey Malivuk said earlier.
"We’ve had a meeting with the Otago councils to discuss the weather possibilities for the region, so they’re prepared for the conditions."
Otago Regional Council duty flood officer Sam Yeo said a "cautious outlook" was being taken because the heavy rain might cause flooding — particularly on the Taieri.
Based on the rainfall forecast, higher focus would be on the Silver Stream catchment (including the Gordon Rd spillway, near Mosgiel), and the Riverside Rd spillway downstream of Outram, Mr Yeo said.
"This is an early heads-up that while we hope no overtopping will occur, people should still prepare and remain watchful and mindful that surface flooding and localised overtopping could occur, particularly overnight Monday and into Tuesday morning.
"We have staff rostered and ready and will be closely monitoring the situation."
In an update about 4.45pm ORC duty flood officer Andrew Welsh said: “River levels are expected to rise and some may come close to channel capacity. If rain falls as forecast, rivers are likely to be contained to their channels, but the situation can change rapidly.”
Other areas under close watch include the Water of Leith and the Lindsay Creek catchments, the Pomahaka River in South Otago and rivers in North Otago.
Mr Malivuk earlier said it was possible a heavy snow warning would be issued for the Leith Saddle at the top of the Dunedin Northern Motorway this morning, and streets in the higher suburbs of Dunedin would be affected.
"So far, it’s been a pretty benign winter for you guys, but it’s going to be a busy few days for Otago and Southland.
"I think you’re going to see just about every weather phenomenon in that time."
MetService this evening updated its heavy rain watch for Southland and Clutha (near and northwest of Raes Junction), from 8pm today until 3am tomorrow, when rain might approach warning criteria.
There was a chance the watch would be upgraded to a warning.
A heavy snow watch for inland parts of Otago and Southland, as well as eastern Fiordland, was also updated, from 8pm today until 6pm tomorrow.
MetService said rain could turn to snow above 200 metres where heavy snowfall is possible. Amounts may approach warning criteria.
A heavy snow warning was also in place for South Canterbury (including the foothills) for all of tomorrow, when rain was expected to gradually turn to snow above 400m.
Mr Malivuk said heavy snowfall was possible above 500m, where amounts might approach warning criteria, and there was a chance the watch would be upgraded to a warning.
A strong wind watch was also in place for Fiordland, from 8pm today until 8am on Wednesday, when southeast winds may approach severe gale in exposed places.
This may damage trees, powerlines, and unsecured structures, and driving may be difficult, especially for high-sided vehicles and motorcycles, he said.
Up to 15cm of snow was expected on the Crown Range Rd above 700m, between 6am and midnight today, and further snow is expected tomorrow to even lower levels.
On the Lindis Pass (SH8), 2cm-5cm of snow was expected above 700m, between 6am and midnight today and further snow was expected tomorrow to lower levels.
The Milford road (SH94) east of the tunnel could expect up to 6cm to accumulate above 800m between 6am and 8pm today.
Light to moderate rain was expected for the rest of tomorrow and Wednesday morning.
He said there was a "chance" any of these weather watches and warnings could be upgraded to a red warning, and urged residents to be wary of streams and rivers rising rapidly, surface flooding, slips and difficult driving conditions.
He urged people to clear drains and gutters to prepare for heavy rain, avoid low-lying areas and drive cautiously.
