The mercury has already climbed past 30 degrees in parts of the Illawarra as the region tracks towards what could be its hottest October day on record.
It reached 30.9 degrees in Albion Park and 30.1 degrees in Kiama just after midday while Nowra temps spiked at 33.4 degrees at lunchtime, according to the Bureau of Meteorology (BOM).
A maximum of 38 degrees is predicted to be reached between 4pm and 5pm today.
Bureau of Meteorology NSW Weather Services Manager Andrew Treloar said temperatures would remain high until a strong southerly buster came through around 9pm.
He said a strong cold front was drawing in hot, dry air from the north west, pushing the mercury up.
‘‘It’s the air ahead of the cold front that brings all that hot air down,’’ he said.
‘‘We’re in the head part at the moment - that’s what’s dragging down those hot north westerly winds.’’
He warned the southerly buster would bring strong, potentially damaging winds to the coast.
Wollongong residents have already taken to the beach to cool off:
North Beach busy with people escaping the heat today don't forget to be sun safe in todays heat onour beaches pic.twitter.com/jOpQu22pn2— Wollongong City (@Wollongong_City) October 10, 2013
Sydney's Observatory Hill hit 30 degrees at 10.15 this morning.
Forecasters said Sydney could record 39C later today, with firefighters on standby statewide to tackle potentially catastrophic fire conditions.
Fire and Rescue NSW said it would have almost 250 fire engines and 1000 firefighters working during the day after mobilising reserves and recalling off-duty personnel.
Sydney, the Blue Mountains, the Illawarra region, the Central Coast and Newcastle are expected to be the worst affected areas.
Ambulance NSW’s Jackie Levett said there had already been 24 cases of people fainting by late this morning - a higher rate than usual, and most likely due to the heat.
People should keep hydrated throughout the day and check up on elderly neighbours, she said.
Extra paramedics have been rostered on ahead of the high temperature forecast.
All tracks and trails in Sydney’s national parks have been closed as well as some vantage points for the departure of the International Fleet Review tall ships, the Office of Environment and Heritage says.
The Bureau of Meteorology also forecasts gusts of up to 100 km/h, while the southern ranges could see winds up to 120km/h.