Sydney is experiencing its hottest day in more than two years, with temperatures reaching 40.1 Celsius at Penrith.
The Harbour City reached a maximum of 37.9C. Its last day above 35C was January 26, 2021, with the intervening period its coolest spell in 115 years, with 768 days below 35C.
Western Sydney also recorded its hottest day in more than two years.
Penrith's temperature was 13C above average, the highest for March in 25 years, ABC meteorologist Tom Saunders said.
Bureau of Meteorology senior forecaster Gabriel Branescu said the city could expect the heatwave to continue until Wednesday.
"We have some hot conditions developing," Mr Branescu said.
"There's a heatwave warning for New South Wales and the conditions will affect areas from the Queensland border down to Sydney."
Other parts of the state also experienced above-average maximum temperatures, including:
- Badgerys Creek: 40.4C, which was 14C above average and the hottest day in 26 months. It was also a March record
- Sydney Airport: 40.6C, which was 15C above average and the hottest day in 26 months
- Newcastle: 36.2C, which was12C above average and the hottest day in 28 months
- Wollongong: 38.4C, which was 15C above average and the hottest day in 28 months. It was also a March record
Mr Branescu said the hottest days would be Monday and Tuesday, with the city predicted to reach 34C tomorrow, before the conditions drop to the mid-to-late twenties from Wednesday.
A total fire ban has been declared from midnight for the Greater Hunter which includes Newcastle, Lake Macquarie and Port Stephens.
It also applies to Bathurst, Blayney, Cabonne, Cowra, Lithgow, Oberon and Orange in the central ranges council area.
Surf Lifesaving NSW chief executive officer Steven Pearce has warned beachgoers to swim between the flags.
"We imagine beaches will be very packed up and down the coast today," Mr Pearce said.
"We've had 28 drownings over summer, all of which were swimmers outside the yellow and red flags.
"Swim somewhere where lifeguards are on duty. If we're not there, we can't see you and we can't save you."
The warning comes after an 18-year-old boy drowned on the weekend at Sydney's Northern Beaches after bodysurfing at Freshwater Beach with his friends on Saturday.
His body was found the following morning.
Bushfire threatens properties
The Rural Fire Service (RFS) said that about 50 properties were at risk from an out-of-control bushfire in the state's Central West.
That blaze — which broke out on Sunday afternoon at Tambaroora, near Hill End, about 80 kilometres north of Bathurst — and has since burned more than 170 hectares.
It has been upgraded to Emergency alert level for people on Ullamalla Road, Alpha Road and the western side of Hill End Road.
Those people are being told to seek shelter as the fire approaches because it is too late to leave.
An extreme fire danger warning has been declared in three regions and total fire bans are in place for seven areas, with hot, dry conditions — combined with gusty, north-westerly winds increasing the risk.
This will see 34 schools closed in the Central West near Mudgee, in Bathurst and Orange Lithgow and Oberon.
Most inland centres will have top temperatures in the mid to high 30s, while Dubbo and Bourke could both nearly reach 40C.
Nyngan, Coonamble and Brewarrina are forecast to hit 40C and Walgett and Bourke 41C.
