With a temperature of 36C forecast for Christchurch today, residents are being urged to keep hydrated and stay sun-smart.
Meanwhile, firefighters have helicopters on standby.
North easterlies are bringing hot temperatures to the city with a high of 29C yesterday, and 32C forecast for Wednesday.
The record January high for Christchurch was 35.9C in 1979.
Canterbury Medical Officer of Health Dr Ramon Pink said while the hot weather is welcome, overheating is a condition that can be 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 and people suffering from chronic, acute and severe illness," Pink said.
People should avoid going outside during the hottest time of the day, wear lightweight, loose-fitting clothing and stay in shade when possible, he said.
"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 - slip, slop, slap and wrap 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."
Anyone unable to cool their home could consider spending a few hours in an air-conditioned public building like a library or mall.
North Canterbury principal rural fire officer Bruce Janes said the fire risk was extremely high, with strong winds expected today.
Yesterday, five houses were evacuated at Pines Beach after a large fire spread and came within metres of the properties. Firefighters yesterday said they were throwing major resources at it while the winds were favourable, out of fear today's gusts could blow the fire further.
Another fire, in a gully on Balmoral Hill, also spread quickly yesterday, and came within metres of houses, but firefighters were able to put it out during the afternoon.
Canterbury is in a restricted fire season, where any open-air fire needs a permit.
Janes said really hot weather combined with strong winds is a lethal combination.
"People need to get out and check their old burns over and over, if you've had a big burn off in the last couple of months you need to get out, dig it up and over and saturate it.
"Until Thursday, when the rain comes, people should avoid any lawn-mowing and hay making unless you are 100 per cent sure the paddock is clear of stones."
Janes said any jobs that cause sparks, including welding or construction, should be avoided.
"We will have three helicopters on immediate standby so they are ready to lift straight away, so yes the level of alert is very much heightened.
"We check the weather over and over again, and we really focus on the wind strength and humidity," he said.
Around the country
Whangārei: A high of 29. Cloudy periods, and a warm day. Light winds and afternoon sea breezes.
Auckland: Partly cloudy, with southwesterlies and a high of 25.
Hamilton: A high of 27. Mainly fine, some morning and evening cloud. Southwesterlies.
Tauranga: High of 31. Fine and warm. Light winds and afternoon sea breezes.
Rotorua: Fine and warm. Light winds and a high of 28.
Napier: High of 26 and fine. Light winds and afternoon sea breezes.
Whanganui: Morning cloud clearing to fine. Northwesterlies and a high of 25.
Wellington: A high of 23 with cloudy periods. Northerlies picking up in the afternoon.
Nelson: Fine. Light winds and afternoon sea breezes with a high of 26.
Dunedin: High of 30. Fine and hot with some high cloud. Northeasterlies, turning gusty northwest around midday.
Queenstown: Fine with some high cloud. Gusty westerlies, with a high of 26.