NSW's average maximum temperature is expected to top 40C for the next few days after some towns already exceeded 47C.
Authorities have warned people to think about coping measures as a low-pressure trough slowly crosses the state between Tuesday and Sunday morning.
It helped Hay Airport in the Riverina crack 47.7C on Tuesday afternoon while Lower Western town Mulurulu also made it to 47C.
Some 17 other weather stations in the state's west also reached at least 45C.
NSW Health environmental health director Richard Broome urged people to heed warnings about the heat over the coming days and take preventive measures.
He said deaths increased 13 per cent and hospital emergency presentations increased 14 per cent during a similar heatwave in 2011.
"These kinds of conditions can and do have a serious effect on people's health," Dr Broome told reporters on Tuesday.
"It's vitally important people heed these warnings and take some really simple precautions to protect themselves.
He said people need to plan their day around the heat by doing any necessary outdoor work first thing in the morning and remaining indoors at the hottest times of the day.
People are also advised to avoid physical activity, stay hydrated throughout the day and to check on vulnerable friends and neighbours.
"If it's passed your mind to check on neighbours, just do it," NSW Police Deputy Commissioner Jeff Loy said.
Authorities are particularly concerned about areas west of the Great Dividing Range, where maximum temperatures in some areas will exceed 45C and nights will be "oppressively hot".
Sydney's west is expected to exceed 41C on Wednesday, Thursday and Friday but the city and coastal suburbs should escape the worst of the conditions.
But Bureau of Meteorology forecasting manager Jane Golding warned the humidity will prevent the coastal areas from cooling too much each night.
She said southwestern NSW will start to cool on Friday while the cooler air should eventually reach all of Sydney by late on Saturday.
