A TODDLER is undergoing surgery after a falling tree branch crushed his leg.

The two-year-old boy was at Apex Park, on Forest Ave, Hawthorndene, when the accident happened.

He was taken to the Flinders Medical Centre with leg injuries.

A Flinders Medical Centre spokeswoman would not comment on his condition, but said he was in surgery.

The accident prompted warnings from the State Emergency Service, which attended more than 178 jobs as of 10pm as winds of more than 100km/h lashed the state.

SES public relations manager Jon Carr warned the public to be particularly cautious around trees, especially when it was so windy.

"We're advising people not to camp underneath them, or shelter underneath them or park their cars underneath them," he said.

"What's happened is they (trees) got stressed in the heat - it can apply to young and old trees. Trees can drop branches at any time without warning."

Mr Carr said he expected emergency services would be busy overnight as winds moved over metropolitan Adelaide and the north of the state to towns including Port Pirie, Hawker and Leigh Creek.

"(There has been) no major damage at this stage ... trees down, that's mainly the sort of jobs," he said.

The Bureau of Meteorology issued a severe weather warning for Adelaide, the Mt Lofty Ranges, Yorke Peninsula, Flinders, Mid North and parts of the North East Pastoral districts.

Strong winds meant Country Fire Service volunteers were on high alert near the remains of the Eden Valley and Keyneton fires.

Volunteers will continue to monitor the region closely over the next week as temperatures remain in the high 30s.

Total fire bans have been declared in four districts - northwest and northeast pastoral, Flinders and the Mount Lofty Ranges. A severe fire danger rating is in place.

SES Chief Officer Chris Beattie said people should never park their car or shelter under large trees in very hot or windy weather.

"It's also important that motorists drive to the conditions," Mr Beattie said.

"If you can't avoid travelling during times of strong winds, then make sure you keep a close watch out for a fallen tree, branch or other debris that may be on the road. You never know what could be around a corner."

The cool change swept through overnight giving South Australians some welcome relief from the weekend's horrendous heat.

Australia's stifling heatwaves are threatening to make summer Australia's deadliest season, experts have warned, as Adelaide sweltered through its hottest February day on record Sunday.

Sixty-six people affected by heat stress have presented to South Australian hospitals since Thursday and 10 have been admitted for treatment during a stifling run of 40C-plus maximums, including Sunday's 44.7C at 2.30pm. The previous February record in Adelaide was 44.3C on February 14, 2004.

So intense was the heat blast that Mount Gambier, in the traditionally cooler South East, recorded it highest temperature of 44.9C.

A top of 29C is expected today after a noticeable drop of more than 20C overnight had many waking to much more manageable conditions. By 9.30am it was a mere 23.3C.

The sweltering start to the month follows the hottest January in 13 years - the fourth-hottest on record - when bushfires raged across the state and more than 300 people presented at hospitals with heat-related illness.

Winter has been the most common season for deaths for more than 100 years, but scientists have projected that temperature increases will make summer deaths more common within the next few decades.

More: New record for hottest February day

"There used to be about a 30 per cent difference in the proportion of people that died in summer compared to winter. Over the 40 years we studied, it's halved," said Dr Charmian Bennett, a researcher from the Australian National University.

"If the trend continues, there will be equal numbers of summer and winter deaths in around mid-century," she said.

More people die in winter months than in summer from causes linked to diseases such as asthma, heart disease and pneumonia and seven more South Australians die every day in August compared with January or February, according to ABS data from the past ten years.

Businesses were closed and sports events - including Athletics SA meets and junior cricket matches - cancelled the mercury again climbed well beyond the expected top of 41C.

More: Bushfire danger despite cool change

SA Health chief medical officer Professor Paddy Phillips said the risk would continue even as conditions eased early this week.

"The highest risk period during a heatwave is typically after the third day when prolonged high temperatures both during the day and at night offer little relief from the heat, Prof Phillips said.

"While temperatures are set to decrease over the next couple of days, it will take a little while for houses and the community in general to cool down, so people should still be on the lookout for signs of dehydration and heat stroke."

The owner of Conversations Cafe at Gawler, Mignon McLeod, was admitted to hospital on Saturday suffering heatstroke after becoming ill while working in her kitchen.

"I won't be opening today as I am at this moment in hospital with heatstroke from my hot, hot kitchen which sadly has no air con," she wrote on the business Facebook page.

"Please take care (of) yourselves and keep cool and hydrated as you don't know with heatstroke until it's too late."

Although temperatures this week are expected to stay under 40C, they are forecast to reach the high thirties later in the week and the State Emergency Service has urged the people to seek shelter.

"People need to take precautions and continue to check on the elderly and vulnerable," SES chief officer Chris Beattie said.

Surf Life Saving SA issued their own warning about the heat and urged the public to "play it safe and not take unnecessary risks".

Small businesses in the retail, construction and maintenance sectors are most likely to suffer economically from the effects of the heat, according to Business SA.

Small operators in building and construction will be forced to work harder in the cooler months to make up for days off the job, while retailers are reporting slower trade as customers opt for the comfort of air-conditioned homes.

Business SA director of policy Rick Cairney said heatwaves of the type seen in the early part of this year can have an adverse effect on the cashflow of businesses across a number of sectors.

"For every business, cashflow is extremely important and especially for small business," he said.

"If you have a heatwave, five or six days in a row of hot weather, then of course it has a detrimental effect."

"People seem to bunker down and that's not good for bricks and mortar retail when people just don't go out, and obviously in the construction industry it has an impact because a lot of the industry stops working when the temperature hits 38 degrees. My understanding is that trades like landscapers are the same."

However, Rundle Mall Management Authority chief executive Ian Darbyshire said strong foot traffic had been reported by mall traders throughout the hot spell.

"We get the odd day when the hot weather affects us but it hasn't been too bad," he said.

"A lot of people cool down in the department stores and centres along the mall. I was down there yesterday and it was fairly busy - a lot of traders were telling me they've been doing very well."

"After a couple of days sitting inside your home, why would you continue to burn your own electricity when you can go into a food court and watch the world go by - we're social animals after all."

- with Tom Dougherty

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Originally published as Branch falls on boy in fierce winds