Mercey said the south would see temperatures in the 30's again but it may be cooler thanks to some breezes and it would become cooler still by Wednesday.
Weatherwatch reported a high of 35C in Edievale, Otago.
The hot weather resulted in a significant drop in the flow of the Whitestone River, prompting Fish and Game Southland to rescue fish including brown and rainbow trout, adult lamprey, upland bullies and long-fin eels on Monday afternoon which were marked to be released further down the catchment, Fairfax Media reported.
In Invercargill the Salvation Army Family Store even resorted to closing early because of the "extreme heat".
Following a milder start to the week, a frontal system moved towards the country from the west bringing strengthening winds and rain to various regions over the coming days.
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".
On Wednesday western parts of both islands are expected to see rain, with Nelson, Westland and Taranaki being 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 looks to be a wet week for many, eastern regions in the south island remain 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.
The hot weather was also affecting fruit and vegetable growers, prompting a call from Horticulture New Zealand president Julian Raine to build more dams so growers would not have to kill off their own crops, Radio New Zealand reported.
On Thursday most of the north island would see the rain continue and the front moves east, wet weather will begin to ease by Friday.
The frontal feature was also forecast to strengthen northerly winds as it spreads over the country, particularly about coastal areas. These winds could become strong in exposed areas.
Tuesday'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
Napier Cloudy then fine in morning, High 24C Low 17C
Wellington Cloud with showers then fine, High 25C Low 16C
Christchuch Morning cloud then fine, High 26C Low 16C
Dunedin Fine then showers in afternoon, possibly heavy, High 28C Low 18C
Queenstown Fine with gusty northerlies, High 26 Low 15C
