"People are advised that this heavy rain is likely to cause rivers to rise rapidly and may produce localised surface flooding and slips."
MetService meteorologist Peter Little said he expected that very few places would miss out on rain.
It was good news for farmers on the West Coast, where a near-record dry spell should come to an end, he said.
"It's not often that folk on the coast want rain, but after three weeks of unseasonably dry weather this rain will be welcomed by many."
As the trough crosses the country the moist northerly flow would be replaced by cooler southwesterlies.
"By the end of the weekend the rain should have cleared as a ridge builds behind the trough, though a few showers will linger in the west," Mr Little said.
"However, another rain bearing trough is readying itself to move onto the country early next week, so the more unsettled weather pattern we expect during springtime looks to have finally arrived."
WeatherWatch.co.nz analyst Philip Duncan urged motorists to drive with care, especially those heading out of Auckland for the weekend later today, and motorists in Northland across today.
Heavy rain was likely to move across Auckland later today, but it would not linger, he said.
"For Auckland rain will most likely clear by midnight."
Weekend forecasts:
Whangarei
• Today: rain with strong north east winds, easing later. High 18C, low 13C;
• Tomorrow: showers, some heavy. North west turn strong south west. High 18C, low 10C;
• Sunday: a few showers clearing. Strong southwesterlies easing. High 17C, low 10C.
Auckland
• Today: rain, with heavy falls this evening and northeasterlies becoming strong, gusting 90 km/h in exposed places. High 18C, low 13C;
• Tomorrow: rain easing to a few showers before dawn with northerlies easing. Winds turning strong south west from late afternoon or evening, gusting 80km/h in exposed places. High 18C, low 10C;
• Sunday: showers becoming isolated in the morning. Strong southwesterlies easing in the afternoon. High 16C, low 9C.
Hamilton
• Today: Showers, evening rain. Northeasterlies freshen. High 18C, low 13C;
• Tomorrow: Few showers, becoming frequent afternoon. Northeasterlies dying out. High 19C, low 9C;
• Sunday: A few showers easing, southwesterlies easing. High 16C, low 6C.
Tauranga
• Today: Periods of rain. Northeasterlies freshen. High 17C, low 14C;
• Tomorrow: Showers, chance heavy afternoon. Northerly dies out. High 19C, low 10C;
• Sunday: A few showers clearing. Southwesterlies developing. High 17C, low 7C.
Wellington
• Today: Fine spells, scattered showers developing this evening. Northerlies. High 15C, low 10C;
• Tomorrow: A few showers and fine spells with northerlies dying away. Rain spreading to most places in the evening with southerlies freshening. High 14C, low 8C;
• Sunday: Rain easing to isolated showers in the morning, clearing later. Southerlies dying out in the evening. High 12C, low 7C.
Christchurch
• Today: High cloud increasing, occasional rain tonight. Light winds. High 17C, low 8C;
• Tomorrow: Mostly cloudy, with rain easing to a few showers, then clearing in the evening. High 14C, low 6C;
• Sunday: Becoming fine early, but isolated showers developing around midday with southwesterlies, showers clearing in the evening and winds dying out. High 12C, low 5C.
Dunedin
• Today: Cloud increasing and occasional rain developing, possibly some heavy falls for a time. Light winds. High 17C, low 6C;
• Tomorrow: Rain clearing late morning, then fine spells increasing. Light winds. High 13C, low 2C;
• Sunday: Mainly fine with northwesterlies, a few morning showers with a brief southwest change. High 12C, low 3C.
Source: MetService
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