The east coast of both islands would be dry with the sunniest weather in the North Island's east, said Mr Duncan.
Cloudier spells were likely on the South Island's east coast.
Mr Duncan said Sunday would bring downpours across the North Island in the early hours, particularly western and central areas.
There would be a mixture of tropical downpours, cloud and light drizzle with very high humidity in the upper north.
As the day went on and the tropical heat increased there might be even bigger downpours developing, he said.
The humidity would ease for areas south of the lower North Island and thunderstorms were possible too.
MetService communications meteorologist Lisa Murray said a westerly flow across the South Island would bring rain to the West Coast tonight, some of which would be heavy.
Those fronts were would also affect the southern part of the east coast of the South Island, which might receive some scattered rain this afternoon or evening.
She said the North Island was mainly fine today with a bit of a cloud but could expect "the odd spot of drizzle" this evening.
Some strong winds were today affecting Manawatu, Wellington and Wairarapa.
Ms Murray said humid air moving over the country would bring overnight temperatures of 20C to 21C in some parts of the upper North Island over the weekend.
The Wellington region and upper South Island would receive overnight temperatures of around 18C and conditions would be muggy and humid.
The warm, humid air would bring rainfall, arriving in western areas tonight.
Ms Murray said the South Island would receive a mixture of wet and dry weather over the weekend and the weather would be fine in eastern areas of the South Island.
Ms Murray said the risk of Tropical Cyclone Winston directly impacting New Zealand was diminishing day by day.
Another tropical cyclone named Yalo had developed near the Cook Islands north of Rarotonga. It was a category one tropical cyclone, which was the weakest type of tropical cyclone and not expected to intensify.
Whangarei: Mainly fine today with some evening cloud. Cloud with light rain on Saturday and periods of rain on Sunday. Highs: Today, 28C. Saturday, 28C. Sunday, 28C.
Auckland: Fine today with a bit of cloud on Saturday and light rain from the afternoon. Sunday will also bring rain at times. Highs: Today, 27C. Saturday, 23C and Sunday, 27C.
Hamilton: Morning cloud was clearing today. Tomorrow humid air would bring some light rain. Sunday would also bring rain at times. Highs: Today, 27C. Saturday, 28C. Sunday 27C.
Tauranga: Fine with light winds today. Saturday and Sunday cloudy with some rain. Highs: Today, 27C. Saturday, 26C. Sunday, 27C.
Wellington: Fine with some cloudy periods today. Saturday would be cloudy with a bit of rain and strong winds and Sunday would be mostly cloudy. Highs: Today, 24C. Saturday, 23C. Sunday, 24C.
Christchurch: Some cloud would burn off today. Tomorrow would be fine with high cloud and Sunday would also bring morning cloud. Highs: Today, 30C. Tomorrow, 31C. Sunday, 23C.
Dunedin: Was receiving some rain through the day which would be clearing in the evening. Saturday would bring a few showers and things would get cooler on Sunday. Highs: Today, 30C. Tomorrow, 25C. Sunday, 25C.
