Demand for ambulances has risen by 40 per cent across the Sydney metro area today, with temperatures in the western suburbs hitting 40 degrees Celsius.
By 2:30pm Penrith had reached 43.4C, while several suburbs in Sydney's west had reached 40C, including Badgerys Creek, Bankstown, Camden and Horsley Park.
Areas in the east were spared thanks to some sea breezes.
Temperatures at Observatory Hill were just shy of 30C.
NSW Ambulance chief inspector Brian Parsell said they expected demand for ambulances to continue to rise throughout the afternoon, and said children and the elderly were most at risk in the hot weather.
He encouraged people to stay indoors and to avoid sports and physical activities.
"Turn on the air conditioning or turn on a fan, make sure you increase your oral fluid intake so that you remain hydrated," Inspector Parsell said.
Ben Scalley from NSW Health said dehydration and heat stroke were common health problems during heatwaves, but there were also increased incidents of other illnesses.
"Heat has a lot of effects on the body in terms of stressing the body and we even see increases in effects like heart attacks during these periods," he said.
A total fire ban has been declared for the Greater Sydney and Illawarra-Shoalhaven regions.
Relief overnight, but not for long
Simon Louis from the Bureau of Meteorology said there would be a little relief from the heat overnight, but it would be short lived.
"The temperatures will bounce back quite quickly, we are expecting fairly hot conditions over the weekend and into the early part of next week," he said.
Mr Louis said Tuesday would be very hot, especially in the west of the state.
"If you think it feels hot in western Sydney, spare a thought for those out in places like Bourke or Dubbo. As always that is a very hot part of the state but they're going to start to see temperatures up in the mid 40s," he said.
"Over the next five or six days we're not expecting the temperatures to get below 20 in western Sydney, even overnight and it's even worse in western NSW, somewhere like Bourke we expect the temperature to be above 26, even in the night time.
"There's a fairly good chance that on the Tuesday we'll see some December temperature records broken."
Look after furry friends
RSPCA NSW chief inspector David O'Shannessy said pets should ideally be kept inside in the air conditioning during heatwaves, but if kept outside should have ample access to water and shade, keeping in mind the shade would move throughout the day.
He advised pet owners to freeze water to put in bowls and paddling pools to help keep pets cool.
"If you can have more than one water bowl that will ensure if one gets knocked over there's a reserve source of water for the animal," Mr O'Shannessy said.
"You can put animals' dry food in with water then throw it in the freezer, it will encourage the animals during the day to keep going back to the ice block in the search for food ... to keep them as cool as you can."
You can even put zinc on your pet's nose, especially if they're a fair coloured animal, Mr O'Shannessy said, and under no circumstances should animals be left in cars.ABC