18/12/2015 11:02 AM AEDT | Updated 15/07/2016 12:51 PM AEST

Apprentice In Critical Condition After Collapsing From Heat-Stress As Adelaide Experiences Another Day Of Extreme Temperatures

Daniel Kalisz via Getty Images
ADELAIDE, AUSTRALIA - JANUARY 13: A man sunabathes in the midday sun during a heat wave at Glenelg beach on January 13, 2014 in Adelaide, Australia. Temperatures are expected to be over 40 degrees celsius all week with health authorities warning the young and elderly to remain indoors. (Photo by Daniel Kalisz/Getty Images)

An apprentice is in a critical condition after collapsing due to heat stress on a building site in Adelaide.

In just his third day on the job the 17-year-old was rushed to hospital on Wednesday after falling victim to the worst heatwave Adelaide has experienced during December since official records started.

Construction, Forestry, Mining and Energy Union state secretary Aaron Cartledge told the ABC the boy was “still in a bad way.”

“He has issues with his kidneys at the moment,” Cartledge said.

“He also looks like he’s got a lung infection from the amount of vomiting he’s actually been doing over the last couple of days.”

Today, temperatures are tipped to hit 42 degrees in Adelaide. The heat is expected to continue over the weekend before cooling on Monday.

The State Emergency Service has issued an extreme heat warning and is urging the public to exercise extreme care during the heatwave.

“There have been significant SA Ambulance call outs and an increase in people presenting to Adelaide metropolitan hospitals, some suffering from heat associated illness,” the SES said in a statement.

“Heat-related illness can result in severe health issues and can be fatal.”

On Thursday afternoon the mercury reached 42.9 degrees and senior forecaster Mark Anolak predicts the extreme heat won’t be easing any time soon.

"I imagine we'll see a temperature rocket up to the mid to high-30s very quickly today and then sort of plateau around about the high-30s and then 40 degrees for most of the day, spiking at 42 degrees in the afternoon," he said.

At 10am local time, temperatures across South Australia were already approaching 40 degrees (Bureau of Meteorology)

"Currently the high over the Tasman Sea is really stationary and that's resulting in hot, dry northerly winds across South Australia."

Melbourne is also experiencing hot December conditions, with temperatures set to hit 37 degrees today.

Both South Australia and Victoria’s fire services have issued complete fire bans and severe fire warnings for several regions.

"We've had hot conditions all week in northern parts of Victoria, the conditions have changed yet again, they are now tinder dry," Emergency Management Commissioner Craig Lapsley said..

"We are now seeing all of the fuels, whether it be grasslands or bush drying to the point where they will carry extensive fire, intense fire, fire that will do damage in the state."

The SES is urging South Australian residents to follow emergency services instructions and has issued a list of precautions people should take.

Among recommendations, the SES said people should make contact with elderly relatives, friends and neighbours to make sure they were okay during the heat.

The SES also said people should stay inside and close curtains and windows during the day, before opening up homes at night if cooler winds came through.

For people without access to air-conditioning or cooling, emergency services recommend visiting public facilities that are cooled, including shopping centres, cinemas and libraries.

The RSPCA has also published a list of instructions to follow to ensure pets keep cool during the heatwave too.