Temperatures in some parts of the South Island soared to 30 degrees by mid-morning today.
Christchurch is expecting temperatures of up to 33°C today, leaving the city council warning people to conserve water or they could face water restrictions.
MetService is predicting the hot weather will continue, with 31° and 30° on Sunday.
The council's head of Three Waters and Waste John Mackie said last weekend the city used half a billion litres of water, the highest consumption since 2009.
Mr Mackie said if high demand continued this weekend water restrictions may be introduced on Monday.
"We're not running out of water per se. It's just the ability to extact that from the groundwater sources ... and get it to the reservoirs. We would like to keep storage at the 100 percent mark.
"We need that storage primarily for fire fighting in the event of a fire emergency."
A water main burst in Lyttelton this morning just as people in Christchurch were being told to conserve water while the city is baking in 30° temperatures.
Mr Mackie said it hampered the Lyttelton Resevoir filling overnight.
The reservoir currently only at about half full, and people in Lyttelton were asked to conserve water today.
For early December, the hot temperatures could break records, the National Institute of Water and Atmospheric Research says.
By 11am, many parts of northern Canterbury were already 30°C. Cheviot was 31.2°, while Waiau was 30.7°.
️️️ As of 11:00am, the temperature was already up to 31.2 degrees in Cheviot & 30.7 in Waiau (North Canterbury) ️️️— NIWA Weather (@NiwaWeather) December 7, 2017
The warmth may reach near record or record levels for December this afternoon.
A MetService forecast says the fine weather was concentrated in the eastern and northern parts of the South Island.
However, rain is on the way towards the West Coast, and could be heavy in parts of Fiordland and southern Westland. Before the rain, the front could also bring strong winds, MetService said.