Other parts of the country will be cooler, but mostly fine, Coutts said.
"Most of the rest of the country is looking pretty good."
Only Gisborne and northern Hawke's Bay catch some showers this week, while it may be drizzly in Fiordland and Stewart Island today.
Wellington will be fine, with temperatures hovering around 20C this week, with Christchurch a few degrees higher.
Today the upper North Island will be mainly sunny, with temperatures for most nudging 25C.
Fresh southerlies will gradually ease today in Wellington, where 21C is expected, while New Plymouth and Palmerston North will be fine with a high of 22C.
And it will be fine across the South Island today, with temperatures ranging from 19C in Greymouth to 20C in Nelson, a pair of 21C highs in Christchurch and Dunedin, and 22C in Queenstown.