Dunedin's mayor says the council is preparing for the worst as a red heavy rain warning gets issued for coastal Dunedin and Clutha.
An orange heavy rain warning was also in place for coastal parts of north Otago with a high chance of upgrading to a red warning.
Warnings and watches have also been issued for other parts of the South Island.
Snow was also expected to build up across parts of the South Island from Sunday morning with orange warnings, heavy snow watches and road snowfall warnings.
The Transport Agency is warning of road closures in Otago and Southland ahead of both the heavy snow and rain.
Council 'ready to go' as heavy rain forecast
"We completely understand the stress that everybody's under," Dunedin's mayor Sophie Barker said after heavy rain warnings were issued.
MetService had upgraded some of its warnings with coastal Dunedin and Clutha going under a red heavy rain warning while coastal parts of north Otago were under an orange warning with a high chance of being upgraded to red.
Any forecast of significant rain was concerning for South Dunedin's flood risk.
"We've done our best to be prepared, and are working hard to make sure that if we get a significant rain event we are all ready to go."
Barker said sandbag stations are opening at the Dunedin Ice Stadium carpark in Victoria Rd, South Dunedin, and Mosgiel Memorial carpark.
Bags, sand and shovels are available on site, but the council was encouraging people to come ready to fill their own bags.
"This is just being hugely cautious, we need to get ready, and if people want to go there and fill up sandbags – stations will be ready.
"We've learnt we need to be [ready] for anything. The weather is challenging."
Barker said contractors have swept low lying parts of the draining network, and mud-tanks have been cleared.
She said she would be monitoring the rain radar and forecasts closely, including looking at the whether for the wider Otago region, and possible flow-on effects.
MetService had said 80 to 120mm of rain is expected in coastal Otago, with the heaviest falls likely from late Sunday afternoon until early Monday morning.
It urged people to avoid unnecessary travel and not drive into floodwaters as dangerous driving conditions and road closures were expected.
Dangerous river conditions, flooding, and slips were also likely and advised people to be prepared for self-evacuation if they see rising water
With snow also predicted, Barker said people needed to be cautious on the roads.
"It was pretty icy here last week, we had a lot of black ice, which caught people unawares... people always need to be cautious during winter, because weather is changeable."
Rest of the South Island
Another orange heavy rain warning, from 4pm Monday until 6pm Tuesday, has been issued for Marlborough and inland Canterbury north of Lake Sumner, excluding Marlborough Sounds.
MetService expected 50 to 200 mm of rain about the ranges, and 60 to 90 mm nearer the coast with a moderate chance of turning into a red warning.
A heavy rain watch was in place for Mainland Southland (not including northern Southland above 400m) until 11pm on Sunday.
Inland Canterbury about and north of the Rangitata River (excluding areas covered by warning) was also in for a heavy rain watch from 12pm Monday.
Fiordland north of Doubtful Sound and west of the main divide is also under a strong wind watch until 6pm Monday.
The West Coast south of Westport is set to go under a strong wind watch from 10am Monday.
Caution on roads urged as snow expected to accumulate
The Transport Agency is warning of road closures in Otago and Southland as parts of the region go under heavy snow warnings and watches.
NZTA says crews are preparing to respond and warns that state highways may be closed and powerlines damaged in the storm.
It's advising motorists to carry warm clothes, water and food in case they get stuck, or need to stop and wait for the weather to pass.
They should also ensure they have sufficient battery range or petrol in the tank in case there are detours.
An orange heavy snow warning is in place for northern Southland and Otago, excluding coastal areas, and a heavy snow watch will be in place for South Canterbury south of the Rangitata River from 3am Monday till 3am Tuesday.
MetService meteorologist Maddy Neal said the region could be in for some heavy accumulations of snow at some levels.
With the warning in northern Southland and parts of Otago going into effect from 6am Sunday until 6am Monday, Neal said there were no current "observations" of snowfall.
"As we progress through the day, the snow level is going to lower, which is why we have a warning out because people in some urban areas, including parts of Central Otago, Northern Southland and even potentially urban areas of Dunedin may see some snow later in the day as that snow level lowers down to possibly 200m in places."
She said the chance of the orange warning being upgraded to a red warning was now moderate.
"It's not a red warning, but we could expect potentially some changes as we go through the day [on Sunday]."
Neal also added there was a chance that snow accumulation could reach 1m in areas above 800m.
MetService had also issued road snowfall warnings for Haast Pass (SH6), Lindis Pass (SH8), Crown Range Road and Dunedin to Waitati Highway (SH1).
A warning for Milford Road was lifted at 9am Sunday morning, with warnings for Lindis Pass and Crown Range Road to go until Sunday night. Haast Pass and Dunedin to Waitati Highway will have a warning until Monday morning.
Neal said they had expected rain to turn to snow near the summit of Milford Rd on Saturday night and Sunday morning with a forecast of two to five centimetres of snow with lesser amounts down to 800m.
For other roads under warnings, snow was expected to accumulate throughout the day and night and Neal urged drivers to be cautious especially with the start of the school holidays.
"We just want to make sure that people are driving to the conditions and thinking about changing their travel plans if they're going to be experiencing disruptions from snow."
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