Heavy rain warnings are in force north of Auckland to the Far North with up to 120mm of rain and potentially severe thunderstorms forecast.
MetService meteorologist Karl Loots said the rain was beginning to set in.
"A slow moving front and subtropical air are moving over the Far North, and during the morning we should see some heavy falls and potential thunderstorms."
The whole of Northland, and down to Whangaparaoa and Great Barrier Island, were also under a severe thunderstorm watch, with potential downpours of 40mm/h and isolated severe thunderstorms to 6pm.
The intense falls brought a risk of flash flooding, MetService said.
Wind gales could reach 90km/h about Auckland and Northland marine areas.
Auckland city would escape the heaviest rain but was still in for a wet day, Loots said.
Apart from that, the weather was looking relatively settled across the country.
Much of the North Island would be dry today with just some cloudy periods.
Southern and eastern areas could see a few light showers through the day.
A high pressure system had formed over the South Island, bringing fine weather to the West Coast and just some drizzle in eastern areas.
"But that should ease further during the day."
Similar weather was forecast tomorrow, with more rain in the Far North and Auckland, while weather in the rest of the country eased further.
By Friday weather in the upper North Island will have eased as well leaving a relatively fine end to the week.
"From Friday to Sunday there is not much significant at all," Loots said.
"The country will be dominated by westerlies over the weekend, so fine out east and in central areas, and the Far North will not be too bad either."
Rain, with heavy falls and possible thundery downpours with hail. Easterly becoming strong morning, gusting 90km/h. 15C high, 11C overnight.
Periods of rain, possibly heavy north of Whangaparaoa. Southeasterlies strengthening morning. 14C high, 9C overnight.
Periods of rain from afternoon. Southeasterlies strengthening morning. 13C high, 7C overnight.
Periods of rain developing this morning. Southeasterlies strengthening this morning. 15C high, 8C overnight.
New Plymouth Mostly cloudy. Strong southeasterlies. 14C high, 5C overnight.
Napier Cloudy periods with a few showers. Southeasterlies. 13C high, 8C overnight.
Wellington Mostly cloudy. A few showers, becoming confined to the south coast in the morning. Strong southeasterlies, gradually easing. 10C high, 7C overnight.
Nelson Fine, early frosts. Southeasterly. 12C high, 3C overnight.
Christchurch Often cloudy, a few showers. Southerly dying out. 9C high, 1C overnight.
Dunedin Often cloudy, with a few showers until evening. Northeast breeze. 9C high, 4C overnight.