The extreme levels of rain are because of the slow-moving nature of the southerly front, Metservice meteorologist Nick Zachar says.
"Tomorrow the front will try and move up a bit, but it is still sandwiched between high pressure systems to the east and the west."
Tomorrow it is forecast to slowly move up into the upper South Island, bringing rain to the Buller and Nelson regions – especially the western ranges - and scattered rain to Canterbury.
Coming in behind the rain is a huge southern ocean swell. Open water waves as high as 10m are forecast.
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 tomorrow.
The system will move into the North Island late tomorrow into Thursday 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.
Along with the big waves, the south should also brace itself for a big drop in temperatures tomorrow and Thursday.
Invercargill will drop from a maximum high of 24C today to a maximum high of 17C tomorrow, along with a howling southerly and rain showers.
In Christchurch it will be even more extreme, plummeting from 23C tomorrow to a frigid 12C on Thursday.
Today most of the South Island's east coast and the entire North Island are in for a fine day.
There may be a few scattered showers from Hawke's Bay around the eastern fringes to Coromandel and north of Auckland.
Tomorrow looks like more of the same, but some rain as the front moves further north in the South Island could spill into Wellington and southern Wairarapa tomorrow night.
Thursday and Friday are more uncertain regarding how far north the front travels, but it could bring rain and cloud over the North Island.
Today's weather
Whangarei Partly cloudy with a few showers. Fresh easterlies. 23C high, 17C overnight.
Auckland Fine spells, chance shower. Fresh easterlies. 24C high, 15C overnight.
Hamilton Mainly fine, but chance shower from late afternoon. Easterly breezes. 24C high, 12C overnight.
Tauranga Partly cloudy, chance shower. Easterly breezes. 23C high, 15C overnight.
New Plymouth Morning cloud then fine. Light winds. 23C high, 14C overnight.
Napier Cloudy periods. A few showers from late morning until evening. Northeasterlies. 21C high 13C overnight.
Wellington Morning cloud then fine. Northerlies. 22C high, 15C overnight.
Nelson Rain, gradually clearing. Southerly breezes. 22C high, 15C overnight.
Christchurch Fine, high cloud at times. Northeasterlies. 26C high, 12C overnight.
Dunedin High cloud. One or two showers from afternoon. Northeasterlies, dying out evening. 26C high, 13C overnight.
