Alexandra hit 31.3C and Wanaka hit 30C yesterday. The tiny settlement of Edievale, south of Roxburgh in Otago, was absolutely scorching at 35C.
Auckland and Hamilton were much cooler at 26C, Tauranga at 24C, and Rotorua a "chilly" 22C.
Mercey said the south would have temperatures in the 30s again today but it would feel cooler thanks to a breeze and it would cool down even further tomorrow.
The hot weather resulted in a significant drop in the flow of the Whitestone River in Fiordland, prompting Fish and Game Southland to rescue fish including brown and rainbow trout, adult lampreys, upland bullies and long-fin eels yesterday afternoon which were marked to be released further down the catchment, Fairfax Media reported.
MetService meteorologist Tui McInnes said once a low pressure system reached the country it would spread a "front across the country bringing plenty of moisture and decent amounts of rainfall for some places". Tomorrow western parts of both islands are expected to have rain, with Nelson, Westland and Taranaki the wettest.
A severe weather watch is in place for heavy rain for Westland and Nelson, with the possibility of more regions being added.
While it looked to be a wet week for many, eastern regions in the South Island remained sheltered by the Southern Alps, McInnes said.
Fire and Emergency New Zealand rural regional manager Mike Grant said conditions had been incredibly dry in the south, and soil moisture levels were very low.
"We've had a few millimetres of rain here and there over the past six weeks, but the grass and vegetation is still tinder dry.
"That coupled with the extremely hot weather we've been having means the fire danger is really high. We had an unprecedented run of hot weather lately, certainly for Invercargill the heat has been record-breaking," Grant said.
A total fire ban remained in place for Gisborne, Horowhenua, Rangitikei and Marlborough.
On Thursday most of the North Island will have rain but the wet weather will begin to ease by Friday.
Today's forecast
Whangarei: Cloudy with showers, High 25C Low 20C.
Auckland: Cloudy with showers, High 25C Low 19C.
Hamilton: Fine with morning cloud, High 24C Low 17C
Tauranga: Cloudy with showers, High 23C Low 19C
Wellington: Cloud with showers then fine, High 25C Low 16C
Christchurch: Morning cloud then fine, High 26C Low 16C
Dunedin: Fine then showers in afternoon, possibly heavy, High 28C Low 18C