It is forecast to reach 33 degrees Celsius in Sydney city, and 40C in western suburbs.
On Sunday, a reading of 47.3C was recorded in Penrith by the late afternoon, making it the hottest temperature recorded in Sydney since 1939. It was also the hottest city on Earth.
The Australian and England cricket teams suffered temperatures in the 50s out on the field at the Sydney Cricket Ground.
England captain Joe Root was taken to hospital to be treated for severe dehydration.
The NSW Rural Fire Service (RFS) is again on high alert and fire bans remain in place for Greater Sydney and the Hunter region.
Extra crews have been placed on standby as winds of between 40 and 45 kilometres per hour are forecast.
RFS Commissioner Shane Fitzsimmons said the total fire ban covered areas experiencing very high fire conditions.
"The total fire bans are again in force today for the Greater Sydney environment which includes the Blue Mountains and the Central Coast, but also the Greater Hunter region which is effectively from Newcastle up through the Hunter to places like Scone and Muswellbrook," he said.
However a southerly change is expected to cool things down by Wednesday.
The Bureau of Meteorology's Andrew Haigh said the cool change would begin moving through Sydney late on Tuesday.