"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.
Heavy rain warnings were in force to 6am tomorrow in Westland, Buller, Nelson and the north and west of Marlborough, also for the Tararua Range, the eastern ranges of Bay of Plenty and the ranges of Gisborne.
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 through to 10pm tonight 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.
Today's weather
Whangārei
A few showers, some heavy, especially afternoon. Strong northwesterlies easing evening. 17C high, 9C overnight.
Auckland
A few showers, some heavy, especially afternoon. Strong westerlies, possibly gale in exposed places, easing evening. 16C high, 9C overnight.
Hamilton
Showers, some heavy. Gusty westerlies easing evening. 14C high, 6C overnight.
Tauranga
A few showers, some heavy afternoon. Strong westerlies easing evening. 16C high, 7C overnight.
New Plymouth Showers, some heavy and possibly thundery. Strong northwesterlies easing evening. 15C high, 7C overnight.
Napier A few showers. Strong northwesterlies easing evening. 17C high,~5C overnight.
Wellington Showers, some heavy and chance thundery, becoming confined to the north by evening as gale northerlies ease. 12C high, 6C overnight.
Nelson Showers, possibly heavy and thundery, clearing evening. Gusty northerlies. 13C high, 2C overnight.
Christchurch High cloud with strong northerlies. A few showers at night with a southwesterly change. 13C high, 2C overnight.
Dunedin Rain developing morning, turning to showers afternoon. Southwesterlies developing late morning, then picking up. 9C high, 5C overnight.
