The unseasonably warm temperatures would continue for most of the country for the rest of the week at least.
"These highs we're getting are stretching north to south so they can really pull up into the tropics. I don't know why we are getting more but it's probably because El Nino has faded out.
"I think Auckland had 18C yesterday, which might be a record-breaker, and it's 19C in Christchurch today, which is about 5C above average ... Dunedin is 17C, about 6C above average."
The mild weather had to be kept in perspective though, Mr Duncan said.
"We we are talking about it being warm we're talking about it being warm for this time of year. When it's cloudy, windy and damp, and with the long nights, it's always going to feel like it's cold. But it really is warm for this time of year."
Almost all the main centres would be wet today, except Dunedin where the sun could break through high cloud as the day went on.
Any severe weather was almost all offshore, with only isolated parts of eastern Bay of Plenty and East Cape at risk of heavy bursts of rain as the sub-tropical northerly rolls slowly through overnight tonight, Mr Duncan said.
"When it's slow moving there's more risk of flooding ... so there's a localised risk of flooding east of Whakatane and north of Gisborne."
A switch to a westerly tomorrow and then a return to a high would mean similar damp weather through to Saturday for most, but a big low expected on Sunday or Monday would likely bring bursts of rain and wind across the country.
It was too soon to know who would be worst affected, he said.
Auckland
Today: Rain turning to drizzle. High 19C.
Tomorrow: Rain turning to showers. High 17C.
Hamilton
Today: Patchy rain. High 18C.
Tomorrow: Rain with chance of heavy falls, clearing to showers. High 16C.
Tauranga
Today: Drizzle with patchy rain and the possibility of heavy falls. High 18C.
Tomorrow: Rain, possibly with heavy falls. High 17C.
New Plymouth
Today: Windy at times, with cloud and some showers. High 18C.
Tomorrow: Periods of rain, possibly heavy in the morning and easing to showers later. High 17C.
Wellington
Today: Gusty with patchy rain. High 17C.
Tomorrow: Showers break to mainly fine weather. High 16C.
Nelson
Today: Steady rain with some dry spells. High 16C.
Tomorrow: Periods of rain easing. High 15C.
Christchurch
Today: Drizzle clearing. High 19C.
Tomorrow: Spits of rain turning to showers. High 16C.
Dunedin
Today: High cloud breaking to sunshine. High 17C.
Tomorrow: Cool sou-west change, possibility of rain in the evening. High 13C.