The Bureau of Meteorology has issued a severe weather warning for damaging winds for the Sydney metropolitan, Hunter, Illawarra and south coast forecast areas for Wednesday.

Peak gusts reached 95 km/h at Wattamolla to Sydney's south as the cool change swept in. Sydney Harbour has also had wind speeds reaching 78 km/h. 

Gale force southerly winds were likely along the coastal fringe between Narooma and The Entrance, the bureau said.

Those sustained southerly winds should range from 35-55 km/h before easing to 25-35 km/h later in the morning before turning south-easterly and easing to 15-20 km/h during the afternoon and evening, the bureau said.

Sydney's maximum on Wednesday will be a cool 21 degrees, just shy of the 22 degrees typical for October. The mercury finally dropped below 20 degrees just after 6 am on Wednesday.

Cloudy conditions should then persist for a couple of days with little chance of rain before city temperatures start climbing back to the mid-20s through to early next week.

Temperatures in the city's west should reach the mid- to high-20s each day to next Tuesday, on current bureau forecasts.

Sunday has the best chance of rain, with 1-5 millimetres predicted for Sydney.

Hottest trio

The city sweltered through another day of temperatures more than 10 degrees above average for October, with multi-day records likely to fall across many parts of south-eastern Australia.

"Sydney, Dubbo, Condobolin, Balarand had their hottest trio of days this early in the season on record," Tristan Meyers, a Weatherzone meteorologist, said.

Coastal breezes helped cap Sydney's peak at 32.3 degrees on Tuesday but they failed to prevent inland regions, such as Penrith, reaching a scorching 38.6 degrees.

Melbourne smashed its record maximum for this early in October on Monday, recording 34.4 degrees, and backed it up with an even hotter 35.8 degrees on Tuesday.

Towns only 80 kilometres north-west of Melbourne were facing fire threats on Tuesday amid changing conditions. Temperatures in the city plunged 13 degrees in 10 minutes as the cool change arrived.

NSW fire authorities, meanwhile, were dealing with 42 bush and grass fires as of late on Tuesday afternoon, a spokeswoman for the Rural Fire Service said.

Some 28 of them were not contained, including a blaze near Bredbo, just west of the Monaro Highway in the state's south.

"As temperatures come down, we will see a reduction in the fire activity," the spokeswoman said.

The fire danger ratings will be very high in the state's north-east but low-moderate to high in most other regions, the RFS said.

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