A southerly front that has doused parts of Fiordland with more than 300mm of rain since Monday is slowly moving north.
The front will blow away a warm northwest flow over the South Island, and drop temperatures in places by 10C, bringing rain, potentially gale-force winds and even snow to 800m.
It has been hovering over Fiordland and lower Westland since Monday, trapped by a high to the east and west, dumping 300mm of rain in Milford Sound and 310mm on Secretary Island.
Westland south of Haast is in for another severe rain warning from 8pm to 9am tomorrow, when a further 80 to 130mm is expected. A warning is also in place for Fiordland from 8pm to 1am tomorrow, when another further 40 to 80mm of rain could fall.
The rain has eased and will clear early today on the West Coast, but will hit the east coast through the day and remain as showers through to tomorrow, Metservice meteorologist Micky Malivuk said.
Some of the heaviest falls today will be in the lower South Island and Central Otago today.
Temperatures are set to plummet to the low-to-mid teens as the front moves through the South Island today and tomorrow.
Today Invercargill will reach a high of 13C and Dunedin will reach just 15C. Christchurch will drop from a balmy high today of 24C to 14C tomorrow.
"Those lower temperatures could also produce snow down to 800m to 900m later today and overnight," Malivuk said.
There is also potential for gale-force southerly winds in southern and eastern parts of the South Island.
The North Island will remain relatively unscathed today, as a ridge produces fine weather for central and western areas, and an easterly flow will bring a few showers to east-facing areas including Gisborne, Coromandel, northern Auckland and Northland.
The remainder of the North Island will be mainly fine with just a few isolated showers.
The cold southerly front will push over most of the South Island today and make it as far north as Taranaki by tomorrow.
The system will regress as a northerly flow takes over.
"Temperatures will rise again once that northerly flow develops over the country," Malivuk said.
Coming behind the rain is a huge southern ocean swell. Open water waves may reach as high as 10m.
The heavy swells will drop by the time they reach New Zealand's coast but they could still reach 5-6m from Fiordland down to Southland today.
The system will move into the North Island late today into tomorrow and even Auckland's west coast could be in for 3-4.5m swells. These swells will be accompanied by an easterly flow moving over the North Island.
Partly cloudy with a few showers. Easterlies easing in the evening. 23C high, 16C overnight.
Auckland Sunny spells. Chance afternoon or evening shower. Easterlies dying out in the evening. 24C high, 14C overnight.
Hamilton Fine spells, but chance shower from afternoon. Light winds. 24C high, 11C overnight.
Tauranga Sunny spells. The odd shower from afternoon. Light winds. 22C high, 15C overnight.
New Plymouth Cloudy periods with a chance of a shower. Light winds. 21C high, 15C overnight.
Napier Fine spells. Northeasterlies. 22C high, 14C overnight.
Wellington Cloud increasing and a few showers developing in the morning. Fresh northerlies. 19C high, 12C overnight.
Nelson Cloud increasing, with rain developing at night. Northwesterlies dying out in the evening. 21C high, 14C overnight.
Christchurch Fine at first, then cloud increasing. Rain developing late afternoon. Northeasterlies turning fresh cold southwest in the afternoon. 24C high, 10C overnight.
Dunedin Occasional rain, becoming persistent around midday. Cold southerlies developing early morning, strengthening by afternoon. 15C high, 9C overnight.