Fire danger warnings were bumped to "high" or "very high" as a new heatwave forecast by the Bureau of Meteorology warned most of the state would be hit by a heatwave this week.

The Illawarra will escape the full brunt but inland NSW is set to be blasted with a "severe heatwave" until at least Friday.

"When high maximum temperatures and above average minimum temperatures are sustained over a number of days, there is a build-up of 'excess' heat," said Alasdair Hainsworth, the Bureau of Meteorology's assistant director for weather services.

"The current event shows large areas of southern Australia will reach severe to extreme heatwave conditions."

An extremely hot air mass over western and inland Australia began moving across the south-east yesterday, the bureau said.

The SES urges people to keep fluids levels up and stay in regular contact with elderly relatives, friends and neighbours.