Wellingtonians could expect a few light showers tonight before clearing tomorrow.
In areas such as Horowhenua, Taumarunui and Taranaki the day would stay fine, Mercer said.
Auckland was forecast to be partly cloudy all day with a few isolated showers this evening, and the story was the same in Northland: cloud build up turning to rain in the evening.
Despite the damp, temperatures were forecast to rise. Yesterday's high was 19C in Whenuapai but Auckland was likely to reach a maximum temperature of 22C today and tomorrow.
In the South Island a ridge was moving across and the east / west divide was similar to the north, with a lot of cloud on the east coast and a little bit of patchy drizzle around Southland, Clutha and Otago, Mercer said.
"We're expecting that cloud to stay in the southern and eastern parts."
A "really nice day" was in store for the Canterbury Plains, high country and southern lakes, with early cloud expected to burn off.
On the West Coast the day would start out cloudy before again burning off to become a nice day, Mercer said.
"There will be a few showers around the ranges today and then a little bit of cloud developing around the evening."
Nelson should be fine with some cloud this evening.
The rest of the week was forecast to be similarly mild, with the North Island on an improving trend.
By Wednesday, most of the North Island would be looking at fine and dry days.
"We have this ridge moving on, because we have easterly to south easterly winds we're expecting similar things," Mercer said.
The North Island would be mainly fine tomorrow with some morning and late afternoon showers and South Islanders could look forward to a mainly fine late afternoon and evening.
TODAY'S FORECAST:
Whangarei: Cloudy at times. Southerlies dying out this evening. High 20C low 13C
Auckland: Partly cloudy, chance afternoon or evening shower. Light winds and afternoon sea breezes. High 22C low 12C
Hamilton: Morning cloud then fine, but chance afternoon or evening shower. Light winds. High 24C low 9C
New Plymouth: Fine with south easterlies. High 21C low 11C
Napier: Cloudy periods. A few showers this morning. Southerlies. High 18C low 12C
Wellington: Partly cloudy, chance shower in the south from afternoon. Southerlies. High 16C low 11C
Nelson: Fine with light winds, then clouding over this evening. High 19C low 11C
Greymouth: Fine apart from morning and evening cloud. Light winds. High 19C low 13C
Christchurch: Cloudy periods, becoming overcast this evening. Southerly breezes. High 18C low 11C
Queenstown: Mainly fine, but southerlies and chance shower from afternoon. High 20C low 11C
Dunedin: Often cloudy with occasional drizzle, turning to a few showers in the afternoon. Southerly breezes. High 17C low 11C