"The windier weather basically removes the chance of frosts for many areas also," Mr Duncan said.
"Overnight lows in the South Island may even be in double digits at times over the next 14 days."
In Auckland, overnight lows are expected to be back into double digits by mid-week.
During the day, Aucklanders should enjoy highs around 16C by the week's end, he said.
However, the break from the cold is unlikely last.
"At this stage the computer models suggest the middle of July sees potential for another wintry snap," Mr Duncan said.
The MetService also expected strong northwesterlies to develop over the South Island this week, and said those in the north may want to dust off their umbrellas.
Duty forecaster Richard Finnie said wet weather was predicted for the top half of the North Island this week.
Windy conditions would likely raise temperatures in the south "and it will probably melt quite a bit of snow in the lower-lying areas of the South Island."