The peak of the heatwave looks set to strike on Thursday.
NIWA expects North Canterbury to reach a scorching 37degC today, with Wairarapa predicted to reach 34.
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A 90-year-old temperature record was broken in Wellington on Tuesday after 30.3degC temperatures caused the rail system to ground to a halt.
The records are prompting warnings about safety, after a spate of instances of children being left in hot cars.
Napier woman Chrystal Alatasi found two upset children inside a hot car at a Napier supermarket on Sunday, when the temperature reached 33degC.
"I couldn't believe the amount of heat that came out of that vehicle... it was like opening an oven," she told The Project.
There have been three separate incidents of children being found stuck inside hot cars during this week's heat wave.
Parenting Place spokesperson Petra Bagust told The AM Show parents need to bring their children with them into shops and not leave them in the car.
"It's basically saying yes to the inconvenience and annoyance of having to take the children into every single shop."
And people who spot children left in cars should find a way to get them out if need be.
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"If you see a kid in the car, hover. Notice the time, wait for a minute or two. If it starts to become five minutes get security, get some help.
"I love the face that somebody went into shop and said 'if anybody doesn't get out and into their car soon I'm going to call security', the fact is I think we should get involved, but get involved in a non judgey way."
Newshub reporter Thomas Mead said parts of Blenheim have become too risky to enter due to the extreme temperatures.
"We've been hearing all kinds of things about the fire risk and water conservation required here because of those high temperatures," he said.
"There are some areas here in Blenheim where people are being advised not to go between the hours of 10am and 4pm because of the risk that they could start a fire in some of these extreme conditions."
Water is also becoming risky as pipes run dry in peak hours.
"There are different provisions in different areas, but the general message is the same: 'Please conserve water where you can'.
"There are some areas where they are struggling to water up the pipes to the houses in peak times, particularly when people are coming home from work, they turn on their irrigation in the garden and in the paddocks."
There's a small amount of relief in sight as the South Island is predicted to have cooler weather on Friday.
Expected highs for Thursday: