Sydney has recorded its warmest August day on record, with the city baking in a late winter blast of heat.
The temperature peaked at 28.1 degrees at 2pm near Sydney's Observatory Hill, according to the Bureau of Meteorology.
However, wind warnings have been issued with gusts expected to hit 100km/hour,
A grass fire is burning at emergency level in the city's southwest near Liverpool and more than 500 calls for help have been made to the NSW SES.
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They have included included fallen trees and damaged roofs.
The SES said a pedestrian was almost hit by a window frame near the Spotlight craft shop on Crown Street in Wollongong when it became came loose in strong winds.
A house was damaged when a tree fell through the roof of a home in Lake Conjola near Ulladulla on the NSW south coast.
NSW, Victoria and Tasmania are in the firing line of wind gusts that have already exceeded 100km/h, the BOM said.
Damage was reported last night in Victoria as the winds swept the state.
The roof of a petrol station at Gwynneville in Wollongong was also blown-off according to the SES.
"We're expecting strong winds and severe weather to continue to impact parts of the south coast, Illawarra, north-eastern parts of Sydney and the Central Coast and Hunter," NSW SES Chief Superintendent Dallas Burnes, said.
Maximum wind gusts of 104 km/h were recorded at 11am in Port Kembla in Wollongong, with similar high winds expected across much of the state.
The extreme winds are expected to ease slightly this evening, before picking up again later in the week.
A wind gust of 119 km/h was recorded at Wilsons Promontory in Victoria at 8:08am today.
Mount Read in Tasmania experienced a 156 km/h gust about 5am.
Despite the strong winds hitting the southern part of the country, hot, clear conditions are persisting for central and northern Australia.
The BoM's Senior Meteorologist, Sarah Scully said the windy conditions would return in force across the southern states late on Thursday night.
Meanwhile, temperatures are expected to set to cool in NSW and Victoria tomorrow, with a top of 23 degrees expected in Sydney, and a top of 17 degrees in Melbourne.
Brisbane and southeast Queensland will stay warm, with a top of 32 degrees in Brisbane and 31 degrees on the Gold Coast.
The Northern Territory will continue to bake with Darwin expecting a top of 34 degrees, and Alice Springs set for 35 degrees.