Rain could be heavier on the west coast in spots like the Buller region through to Greymouth.
Temperatures in Auckland would peak at 15C today and drop to 5C overnight.
A morning shower or two would be followed by fine spells and southwest breezes.
A similar pattern of weather was forecast for places further north like Kerikeri, though things looked set to be a little warmer.
The central North Island was in for a similar Saturday.
Heading further south, Wellington was in for a longer dose of rain and a chilly day - temperatures would peak at a frosty 10C.
A forecast for rain was a little more sporadic in the South Island.
Christchurch and Nelson were in for a high of 11C and intermittent showers.
Spots further down the island like Wanaka escaped the rain - though temperatures were nippier, like Wanaka's high of 9C.
Friday night was a chilly one for much of the country.
Mercer said: "We've certainly had quite a few frosts in the South Island over night, especially in inland areas."
Spots like Oamaru and Wanaka dipped down to 0C while Queenstown was a nippy -2.7C overnight.
Mercer said temperatures in the area were still sitting around there around 6am this morning, but would quickly warm up as the morning progressed.
Further north, the Desert Road was 0.4C overnight.
Temperatures would warm up this morning in most areas, Mercer said, though Southerly winds near the bottom of the North Island would prevent temperatures from becoming too summery.
"It's going to be a pretty cool day, especially in those southerlies in Wellington, and up as far as Whanganui."
The southerly winds whipping through these areas were expected to move off the island as the day progressed, clearing for a warmer evening.
