Metservice meteorologist Lewis Ferris said although the forecaster's thermometers are only registering temperatures in the late 30s, reports as high as 40C are not "unbelievable".
"Not all thermometers are created equal. The same air temperatures could be read differently by two different thermometers because they are not all of the same precision or accuracy.
"The station we have at the Timaru airport got up to 37.5C so it's not unbelievable that someone at their house would've got [readings] reaching that 40C mark."
Other parts of Canterbury are also sweltering under mid-30C temperatures.
According to WeatherWatch, Amberly was sitting at 38C as was Geraldine at 2.30pm.
Christchurch was sitting on 31C and Dunedin on 26C.
Temperatures are expected to reach 36C in Christchurch, 34C in Blenheim and 30C in Dunedin.
MetService meteorologist Alwyn Bakker said that is 13C above the Garden City's average temperature for this time of year.
Fire permits have been suspended in most of the South Island.
Permits are granted when an area is in a restricted season, and have been suspended indefinitely in Canterbury.
They have also been suspended in the Nelson District with the exception of Nelson Lakes, Murchison, and Golden Bay West for 48 hours.
Fire and Emergency NZ's response co-ordinator Colin Russell said with the high temperatures and strong winds forecast, cancelling active fire permits reduces the potential for fires getting out of control.
"Please, avoid any spark-generating activities, like grinding and cutting metals outdoors, or using farm machinery. Even mowing the lawn could cause sparks to start an out-of-control fire. Also check previous burn sites for hot embers and if you see a fire call 111 immediately.
"As an organisation we are well prepared to respond to any incidents across Canterbury."
"We have crews on standby and are ready to protect our communities if a fire does occur."
Crews spent this morning continuing to work to contain a large fire which started at Pines Beach yesterday.
Russell said the 31ha fire is now 60 per cent contained, but it is under control.
"We have around 30 firefighters working on the fire, as well as heavy machinery."
"There are still spots at the fire where our people are managing active fires."
Russel said they are likely to be there for at least a couple more days.
Temperatures are set to reach scorching levels in the south.
It was sitting at 24C by 6am at Mt Cook.
Tony Trewinnard of Blue Skies Weather told Chris Lynch of NewstalkZB he would describe it as "stupidly hot".
"It is probably likely to be the hottest day of the summer in Canterbury. It is our first day above 30C this January and possibly for the whole summer."
"It's happening because we've got some very warm air from the interior of Australia that has worked its away across the Tasman."
Temperatures across the South Island have been high so far this week and are expected to continue.
"Christchurch is currently looking at three days in a row with more than 5C above average," Bakker said.
It is considered a heatwave when temperatures are consistently 5C above average for five days.
The sunshine is not expected to last all week, with a front expected to bring a cool change to much of the country from about Thursday.
Canterbury medical officer of health Dr Ramon Pink said while we may welcome a run of hot weather, overheating is a condition that could prove fatal.
"It's especially important to stay out of the sun where possible, avoid extreme physical exertion and ensure pets and people are not left alone in stationary cars.
"While we are all vulnerable to hot temperatures, some people are particularly at risk. This includes the elderly, infants and children, women who are pregnant, people suffering from chronic, acute and severe illness."
Pink said people whose work involves strenuous physical activity outdoors should be particularly vigilant to avoid overheating in hot weather.
"It's important people exposed to hot weather for long periods of time carry water with them and sip at least half a litre an hour, allow for more breaks in the shade, reapply sunscreen every two hours and schedule the hardest work in the coolest part of the day.
"As well as being sun smart, if you have to go outside, everyone is advised to keep their houses cool by closing curtains on windows getting direct sun, opening windows to get a breeze if its cooler out than in, and consider using the cool cycle on heat pumps."
The hot weather follows a bleak summer in Christchurch with long stretches of below-average temperatures.
Warm temperatures have been felt across the North Island as well today, with the east from Tauranga down to Masterton seeing temperatures between 26-32C.
