Sydney region paramedics say they were called to treat eight people for heat exposure or heat stroke, seven of whom were hospitalised, to 8pm (AEDT) on Friday, following the day's high temperatures.
The patients were aged two to 85 years.
In other parts of NSW, three cases of heat exposure were treated in the north between 4.30pm and midnight, with all patients taken to hospital.
Temperatures are expected to hit the high 30s and low 40s in western NSW, western Sydney and the lower Blue Mountains on Saturday.
However at 10am on Saturday it was only 26 degrees Celsius, according to the Bureau of Meteorology.
Authorities are urging people to drink plenty of water, keep cool, take care of others and have a plan over the weekend.
Ahead of the expected sweltering days to come, police have reminded people of the dangers of leaving kids, the elderly and pets unattended in cars.
Emergency services are also warning the public of a very high risk of bushfires.
Rural Fire Service Commissioner Shane Fitzsimmons said firefighters this year had already battled more than 3800 bush and grass fires and he is urging people to develop bushfire survival plans.
"This summer, complacency could kill, but preparation could save lives," he said.
