It is a wild start to the school holidays with heavy showers dousing most of the country, the odd crack of lightning, and snow on the alpine passes.
MetService meteorologist Karl Loots said a narrow rain band that moved over the North Island was responsible for heavy downpours that caused flooding in parts of Wellington overnight.
Roads were closed, buildings flooded and pepole left without power in the capital.
The Paekakariki Hill Rd is closed due to fallen trees, while Grays Rd near Porirua is closed because of flooding.
In Lower Hutt, State Highway 58 Haywards is down to one lane due to a slip.
Fire and Emergency says it was called out to 18 incidents around the region overnight.
"The rain band has exited now but in behind it comes a cold front in the far south, and an unstable westerly flow, with frequent showers and isolated thunderstorms." Loots said.
"Pretty much everywhere will be affected, except for Gisborne, Hawke's Bay, Wairarapa and Canterbury."
Auckland was in for a showery day, potentially heavy in the afternoon, and a high of 16C.
Most of the heavy rain overnight fell in the ranges, Loots said.
The heaviest falls were recorded about Arthur's Pass, Haast and Franz Josef, with about 30mm over the past six hours.
Severe rain warnings were in force to this morning in the eastern ranges of Bay of Plenty and the ranges of Gisborne, parts of the central North Island, and Fiordland.
Heavy rain watches continued today in eastern Bay of Plenty and the Gisborne ranges, Buller, Nelson, the Marlborough Sounds and Richmond Ranges, the Tararua range and Westland.
Canterbury could also be affected by gale force wind gusts up to 120km/h.
There was a moderate risk of thunderstorms for northwest Nelson, northern Wellington, the Kāpiti Coast, Horowhenua, Taranaki and Waitomo Monday morning.
These thunderstorms could be accompanied by localised heavy rain and small hail, strong wind gusts of 90-110km/h and small tornadoes.
Fiordland, Westland and Buller could also see similar thunderstorms before dawn.
In the afternoon a cold front would move up the South Island reaching Christchurch by midnight, bringing a cold southwest change with it.
Any rain above 600m would fall as snow, adding to the road snow warnings already in force Sunday night for the Desert Rd (SH1), Lewis Pass (SH7), Arthur's Pass (SH73), Porters Pass (SH73), Lindis Pass (SH8), Crown Range Rd and Milford Rd (SH94).
The worst of the week's weather was forecast today, but the unsettled flow would continue until Thursday, Loots said.
Tomorrow the cold front would continue up the country, before another cold front arrived Wednesday, bringing rain to the west and southern parts of the South Island.
A ridge of high pressure would spread on to New Zealand from the Tasman Sea late Thursday, and slowly drift north late Friday as a second cold front approaches the lower South Island from the south.
