A front that raced across the country earlier this week had initiated a drop in temperatures this week, MetService said.
MetService meteorologist April Clark said behind this front, rainfall would be significantly decreased - if not ceased - but so will temperatures nationally.
Blenheim would be waking up to a frosty morning, with temperatures at its lowest since last spring.
Eastern coasts would experience the greatest drops with cool air being dragged from the Southern Ocean by uninterrupted southerlies.
MetService said the cool change was set to last over the country for the rest of the week.
Aucklanders will get a mix of sun and cloud today, with a high 19C, while the capital can expect a mainly dry day after a shower or two in the morning.
It will be mainly sunny in Canterbury, but some cloud across the coast could bring an early shower before clearing away.
Enjoy the sunshine while it lasts, because another front is expected to hit the country tomorrow.
"Tomorrow and Friday, another front is forecast to sweep across New Zealand, cementing a cooler and changeable weather pattern which is typical at this time of year," Clark added.
WEDNESDAY WEATHER FORECAST
Northland, Auckland, Waikato, Bay of Plenty: Mix of sun and cloud, may be a shower or two for Northland and Auckland. Highs: 19-21
Western North Island: Mostly sunny, southeasterly winds. Highs: 16-19
Eastern North Island: Mostly cloudy with a shower or two, cool southerly winds. Highs: 16-18
Wellington: Areas of cloud, may be a shower or two in the morning. Southerly winds. High: 15
Canterbury: Mostly sunny, some cloud about the coast may bring an early shower. Light winds then afternoon northerlies. Highs: 15-16
Southland, Otago: Mostly sunny after morning cloud clears, winds are light tending onshore in the afternoon. Highs: 16-18
Source: WeatherWatch.co.nz