"Head back home and sit in the aircon," is the advice from the NSW Rural Fire Service (RFS), with total fire bans in Greater Sydney and temperatures set to climb to 45 degrees Celsius in parts of the city today.
The NSW RFS also warned of a severe fire danger in the greater Sydney region and the Hunter.
"If someone sees a fire that is unattended, report it to Triple-0, don't assume someone else has reported that fire," NSW RFS Deputy Commissioner Rob Rogers said.
The Bureau of Meteorology (BOM) said the western centres of Penrith and Richmond were expected be the hottest areas but there would be no escape in the Harbour City's heatwave.
Thousands of sports fans braved the hot weather at the SCG for day four of the Ashes cricket Test between Australia and England where temperatures reached 41C by noon.
Sydney Water said it had installed 12 free water refill stations at the venue to help people stay hydrated, and encouraged spectators to bring their own drink bottles.
Charities are concerned rough sleepers in the city could be particularly vulnerable, with the mercury tipping 41C at Sydney's Observatory Hill by midday.
People working for Mission Australia's homelessness transport service Missionbeat will be moving some people to cooler locations.
"We're particularly concerned for the welfare of older people who might have pre-existing health conditions and long term rough sleepers," Missionbeat program manager Shane Sturgiss said.
"If anyone is concerned for the wellbeing of someone who is homeless, we encourage them to give us a call."
An Air Quality Forecast alert of poor for the Sydney region has been issued by the NSW Office of Environment and Heritage, as ozone levels are forecast to exceed national air quality standards.
The BOM is expecting a front to bring cooler conditions to coastal areas during the afternoon and through the west in the evening.
Duty forecaster Helen Reed told people to prepare for a hot, dry day.
"We do have a southerly change moving it's way up the coast during the afternoon," she said.
"It's expected to come through Sydney at 3:00pm or 4:00pm.
"It will gradually migrate further inland and that will provide a significant relief to the temperatures overnight."
