Parts of the North Island are being warned to brace for a sub-tropical drenching with heavy downpours so intense the crippling drought may finally be broken.
Heavy rain is expected to hit the country today and last for up to a week.
At the same time the South Island is facing an autumnal cold snap with temperatures in some regions barely registering above single digits.
MetService said a low pressure system was expected to stay at the top of the country dragging humid air down from the subtropics and bringing rain across northern regions.
The downpour is expected to be so intense there was a possibility warnings would be issued for upper districts of the North Island.
Auckland is expected to see rain developing this afternoon, with a chance of heavy downpours. A high of 24C and an overnight low of 18C are forecast.
Weather conditions are similar in Hamilton and wider Waikato region, with a high of 25C forecast.
Further north, temperatures remain warm - in the mid-20s. Kaitaia will see showers and heavier rain from this afternoon.
Gisborne has occasional rain later this evening, while afternoon rain is forecast in both Palmerston North and Wellington.
Rain was also expected to fall over the Bay of Plenty, Gisborne, the Hawkes Bay ranges and the central North Island high country, but not to warning levels.
At this stage there was still some uncertainty about which areas would get the heaviest rain.
MetService said the watch was for the possibility that rainfall accumulations will reach "warning levels'' in Northland, Auckland, Great Barrier Island, Waikato, Coromandel Peninsula and Western Bay of Plenty from today through to Wednesday and Thursday.
"People in these areas should keep up to date with the latest forecasts in case any of these areas [are] upgraded to a full warning or other areas are added to the watch.''
Weatherwatch.co.nz said the rain would finally bring relief to farmers in Northland and Gisborne signalling an end to the drought.
Meanwhile, temperatures in the far south had started plummeting with Dunedin and Invercargill reaching just 14C today and expected to be even cooler tomorrow.
Weatherwatch.co.nz said Christchurch, which has a high in the mid-20s yesterday, was expected to feel a noticeable nip overnight; plunging to 14C before bouncing back later this week.
- Auckland: Rain developing in afternoon, chance heavy. 24C
- Tauranga: Cloudy periods, evening rain, chance heavy. 24C- Whangarei: Showers develop, chance heavy evening rain. 25C
- Palmerston North: Cloudy periods, afternoon/ evening rain. 21C
- Wellington: Strong southerly change early afternoon brings rain. 21C
- Nelson: Rain then showers, clearing in evening. 22C
- Christchurch: Showers until afternoon, then fine spells. 17C
- Queenstown: Morning cloud and showers, then fine. 15C
- Invercargill: Occasional showers clearing in afternoon. 15C