New South Wales Health is advising people to be cautious in the coming days to avoid heat-related illness.
Temperatures are predicted to reach the low 40s in western Sydney and western New South Wales today and tomorrow.
Fire danger is high to very high across most of the state and there's a severe fire warning for the Northern Riverina.
In Sydney city, the Bureau of Meteorology is forecasting a high of 31 degrees today and 34 tomorrow.
ABC Weather specialist Graham Creed says temperatures were already soaring in parts of the state's north-west before 9:00am (AEDT).
"Sydney is in for some hot weather but really the worst of the conditions are out through the western half of the state," he said.
"At the moment at Bourke, and also out around Brewarrina, we are seeing temperatures already of 35-36 degrees."
New South Wales Health says people should stay hydrated and try to remain indoors during the afternoon.
It says the heatwave that affected the east coast in February last year caused nearly 600 Emergency Department visits and 96 deaths.
Police are also warning people not to leave children or animals in cars and to monitor their alcohol intake.
Signs of heat-related illness include dizziness, confusion, vomiting and head aches.
© ABC 2012
14:16 EDT Sunday saw scorching conditions across Adelaide, in stark contrast to the cool weather during the week.