Air New Zealand has issued a warning the severe weather could disrupt some flights.
Showers and rain were forecast across most of the central North Island and top of the South Island today.
Further north in Auckland, Northland and the Bay of Plenty, showers could become heavy late morning, with possible thunderstorms.
Temperatures will be mild however, with Whangārei forecast to hit 19C, Auckland 18C and Tauranga 17C. The mercury drops further south, with Hamilton in for a high of 16C and Wellington 13C.
In the South Island, the West Coast is in for a relatively fine day while the east coast will be "quite unpleasant".
"There will be a lot of low cloud most of the day in eastern areas, with patchy drizzle about the coast this morning spreading over the whole east coast through the day," Mercer said.
Christchurch and Dunedin will struggle to make it into double digits today with highs of just 10C.
The unsettled weather will continue through the country tomorrow.
A weak front moves up the South Island, continuing the drizzle on the east coast.
Cold temperatures are expected across the country for much of the week, with highs in the early teens for the North Island and single digits for the South Island.
A ridge of high pressure is forecast to build on Wednesday in the South Island, bringing periods of fine weather for the latter half of the week.
Meanwhile, the low affecting Taranaki today moves over the northern North Island tomorrow, spreading the heaviest rain there.
Rain will ease by tomorrow afternoon in lower and central areas, and by Wednesday in the Far North.
The ridge of high pressure will build into the North Island by Thursday and Friday, bringing a fine end to the week for most of the country.
Today's weather
• Whangārei
Showers, possibly heavy and thundery, easing in the afternoon. Northwesterlies. 19C high, 11C overnight.
• Auckland
Showers, possibly heavy and thundery, easing this afternoon. Northwesterlies. 18C high, 11C overnight.
• Hamilton
Periods of rain developing morning, some heavy and possibly thundery, easing towards evening. Northerly. 16C high, 8C overnight.
• Tauranga
Rain developing morning, chance heavy and thundery, easing towards evening. Northwesterly dying out overnight. 17C high, 10C overnight.
• New Plymouth
Rain with possibly heavy and thundery falls. Northerly turning southeast overnight. 16C high, 8C overnight.
• Napier
Occasional rain developing morning, clearing evening. Northerlies. 15C high, 6C overnight.
• Wellington
Rain setting in during the morning with possibly heavy falls. Northerlies turning southerly afternoon, then strengthening evening. 13C high, 8C overnight.
• Nelson
Occasional rain developing this morning. Light winds. 14C high, 5C overnight.
• Christchurch
Mostly cloudy with patchy drizzle, becoming widespread afternoon as southerlies develop. 10C high, 6C overnight.
• Dunedin
Low cloud. Occasional drizzle and cold southerlies developing afternoon. 10C high, 6C overnight.
