Freezing conditions and heavy rain linked to an enormous Southern Ocean storm is set to hit the country next week.

It will bring with it a low air pressure system so deep it's regarded as being associated with the "world's largest storms".

The low pressure system will deepen over the Southern Tasman Sea and New Zealand area this weekend before it smashes the South Island with an "Antarctic blast", Philip Duncan of Weatherwatch.co.nz, says.

Meanwhile, MetService has this morning released a list of warnings or closures of South Island roads due to snow and ice.

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Nine roads are currently affected and a severe wind warning has been issued for coastal Southland and Otago.

Duncan says deep low pressure storms this size do occur in the Southern Ocean "from time to time in winter" but this one was different as it was forming in the New Zealand area then scheduled to rapidly deepen in the Southern Ocean on Sunday and Monday.

"The incoming low is significant and will produce severe weather and is forecast to have very low air pressure, potentially into the 940hPa range which is right up there with some of the world's largest storms.

"At this stage it looks as though the coldest portion of this weather will arrive next Monday, Tuesday and Wednesday, mostly over the South Island and lower North Island."

While the upper North Island would get high winds, it should escape the brunt of the storm, he said.

"It will likely be centred much further south of the country and this further south placement means the upper North Island won't be so vulnerable to the coldest air."

The storm as it's predicted to look as it begins to move away from New Zealand on Tuesday. Image / Weatherwatch.co.nz
The storm as it's predicted to look as it begins to move away from New Zealand on Tuesday. Image / Weatherwatch.co.nz

Duncan said it was too early to know which areas would be hardest hit, however the West Coast would be the target of heavy rain, "then bitterly cold south to southwest winds up to gale force in exposed areas, as far north as Auckland possibly, and snow levels once again lowering to sea level in the south around Fiordland and maybe Southland and Otago too".

Highway closures, flight delays and cancellations were all possible next week, he said.

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Meanwhile MetService said SH94, between Te Anau and Milford, would be reopened at 10am due to heavy snow.

Cautions had been issued for State Highway 7, at the turnoff to Springs Junction in Canterbury due to snow and ice.

The warning was also in effect for further along SH7 on the West Coast road Reefton to Springs Junction as well as SH6 Franz Josef to Fox Glacier.

In Otago, caution was also advised on SH1 between Clinton to Balclutha due to ice.

And on SH1 Bluff, there was flooding near the intersection with Nichol Rd and a 30km/h temporary speed was in place.

THE ENORMOUS STORM

• Weds - Fri: Westerly quarter winds. Increasingly milder than average in the east of both islands. Rain returns to the west.
• Weekend: Saturday kicks off with mild north to northwest winds then rain and showers move back into western areas as the low in the southern Tasman Sea now starts to rapidly deepen. A very late south to southeast change arrives in Southland and Otago on Sunday afternoon.
• Monday: Wintry conditions move up the entire South Island with snow to low levels in Southland, Otago and parts of the West Coast. Wintry in the lower half of the North Island but cold and gusty southwest winds spread across NZ. Road closures possible. Some flight delays possible.
• Tuesday: Another wintry day with snowy conditions and gales in the south of the South Island. Windy and cold in many other regions. Road closures possible. Some flight delays possible.
• Wednesday: Conditions improving slowly but still colder in many parts of both islands.