The main highway between Sydney and Canberra was cut by a bushfire and parts of New South Wales reached 40 degrees Celsius in September for the first time on record.

The New South Wales Rural Fire Service (RFS) said the blaze at Paddy's River, near Wingello, crossed the highway but the threat to property had eased. It was downgraded to Advice level at 6:00pm.

After several hours of closure about 160 kilometres west of Sydney and east of Canberra, the Hume Highway has reopened.

A total fire ban will be in place again for many parts of NSW tomorrow.

Meanwhile, temperature records are being broken as the east coast swelters through a heatwave.

In Sydney's west, it was 36.4C at Richmond, and records were set in other parts of the state.

The grassfire at Paddys River near Wingello is causing lengthy traffic delays ( Twitter: Adam Mobbs )

Northern Victoria and south-western NSW experienced above-average temperatures, with hot north-westerly winds turning gusty west-south-westerly on Saturday afternoon and bringing possible dust storms and damaging winds.

The hottest September day on record in New South Wales was set in 2004 at 39.6C, however Andrew Hague from the Bureau of Meteorology (BOM) said several towns had reached 40C and beyond.

"Wilcania airport has reached 40.5C, Whitecliffe has reached 40.3C, Delta has reached 40.2C and Bourke 40 degrees," he said.

Hot and dry conditions also affected Queensland, where Birdsville reached a top of 41.6C, while in Victoria, Mildura is expected to record its hottest September day on record, reaching 37.7C.

In Queensland, Thargomindah had its hottest September day on record, hitting 40.4 degrees while everywhere west of Roma experienced temperatures 12 to 14 degrees above average.

The heat is expected to push further east on Sunday ahead of a trough, with more temperature records likely to fall.

The mercury at Tibooburra, in north-western NSW, reached about 39C and Mavis Jackson from the Granites Hotel told ABC News it was miserable and dusty.

"Very, very dusty even inside on the floor blowing in at the moment, it's all dust so it's not a real pleasant type of atmosphere here just at present," she said.