Don't leave the house without a raincoat.
The top of the North Island is expecting periods of rain throughout the day, with heavy rain watches issued for places including Northland and Auckland.
Northland will get periods of heavy rain from late this morning through to tonight.
The rainfalls may approach "warning criteria" mainly to the east, MetService says, and people in the area are being told to stay updated with the weather forecast as a result.
There is also a heavy rain watch for Auckland, Great Barrier Island, the Coromandel Peninsula and the Bay of Plenty - west of Whakatāne, including Rotorua.
Aucklanders can expect showers turning to rain this morning before possibly heavier downpours from this afternoon. A high of 12.5C is forecast in the city.
There is also a strong wind watch for the City of Sails as well as for Great Barrier Island.
"Northeast winds may approach severe gale in exposed places at times, mainly north of Auckland Airport," MetService said.
In other parts of the North Island, those in Palmerston North can expect a mostly fine day with increasing high cloud and a significant high of 17C.
Anyone in Wellington can look forward to beautiful conditions and some cloud developing later this evening. A high of 15C and overnight low of 9C is on the forecast.
The gloomy conditions are a stark contrast to the sunny weather seen in those places over the weekend and comes as a complex subtropical low moves over the top part of the country today.
MetService meteorologist April Clark said New Zealand would say goodbye to the calm weather experienced over the past few days as the low moves to the northeast, spreading rain over the country this week.
The National Institute of Water and Atmospheric Research said the weather system was coming from Fiji and Vanuatu.
"While it won't feel like Fiji or Vanuatu in New Zealand - that's where our air is coming from Tuesday to Thursday.
"The result will be some heavy downpours, gusty winds and a few thunderstorms," Niwa said.
The low is due to move over the rest of the North Island tomorrow.
"These features should bring northeasterly gales and periods of heavy rain to northern parts of the North Island," MetService said.
"Also note, due to higher than normal tides over the next few days, there is a possibility of coastal inundation in areas exposed to strong northeast winds.
"People are advised to keep up-to-date with the latest forecasts in case any of these watches are upgraded to warnings, or further areas are added."
THE SUNNY SOUTH:
Meanwhile, those in the South Island will bask in sunny weather today.
Greymouth is looking particularly lovely with fine conditions expected and a high of 15C.
Other areas - Nelson, Christchurch, Dunedin and Invercargill - are all in for sunny weather save for some cloud and the chance of a shower in Invercargill.
Chilly temperatures are, however, also on the cards. Temperature highs range from 4C to 7.5C in those parts.
Only Queenstown sees a somewhat more severe drop in temperature. At 7.15am, the temperature was 0.8C.
- Kaitaia: High: 18C, overnight low: 12C. Rain, possibly heavy, easing to showers this evening. Northeasterlies strong at times this morning.
- Whāngārei: 17C, 14C. Rain, heavy at times, easing to a few showers this evening. Northeasterlies.
- Auckland: 16C, 13C. Showers turning to rain, possibly heavy from this afternoon. Northeasterlies becoming strong in exposed parts.
- Hamilton: 15C, 12C. Cloud increasing. Rain developing this afternoon, persistent from this evening.
- Gisborne: 16C, 11C. Cloud increasing. Rain developing this evening.
- Tauranga: 16C, 13C. Cloudy periods. Rain developing this afternoon, possibly heavy later. Northeasterlies strengthening this afternoon.
- New Plymouth: 15C, 11C. Cloud increasing. Rain developing this evening. Easterlies strengthening.
- Palmerston North: 17C, 10C. Fine with increasing high cloud.
- Wellington: 15C, 9C. Fine with some cloud developing this evening.
- Blenheim: 16C, 7C. Fine, then cloud increasing from afternoon. Light winds.
- Christchurch: 15C, 5C. Fine apart from morning fog.
- Queenstown: 14C, 0C. Fine apart from some morning cloud.
- Dunedin: 15C, 7C. Fine. Some cloud tonight.
- Invercargill: 16C, 3C. Morning cloud and chance of shower, then fine.