The NSW Rural Fire Service has warned any bushfire that started had the potential to threaten lives and destroy homes.

Melbourne Cup day is set to be a long, hot one, with the oppressive conditions unlikely to subside until the evening when a late southerly change hits.

A total fire ban is also in place across large parts of NSW, including the Sydney, Hunter and the Central Ranges, Upper Central West Plains, and Lower Central West Plains.

Warm and muggy conditions were already apparent at dawn today, the temperature just under 20 degrees and humidity at about 90 per cent.

The Bureau of Meteorology had originally forecast temperatures to rise as high as 35 degrees across Sydney, but senior forecaster Jane Golding said this may need to be revised higher.

"Since that forecast went out we've got data back from the balloon we sent out at the airport ... we are still on track to get those numbers and it might even bump up to 37 towards the metropolitan area," she said.

The blistering conditions were the result of hot north-westerly winds being triggered by a low pressure system to the south.

"[The low pressure system] means hot air from the interior of the continent is being dragged towards us," Ms Golding said.

It was the same weather system that brought hot weather to Melbourne and Adelaide yesterday, she said.

Punters were likely to still be feeling the heat hours after today's racing festivities had concluded.

Relief was not likely until about 8pm, when a gusty southerly change was expected to hit Sydney, Ms Golding said.

"If it comes through at 8pm, it will probably still be about 35 degrees at the time and then will drop to about 26 in the first hour, so it will be a pretty rapid drop."

Despite today's oppressive conditions, no records were under threat.

Cooler and wetter conditions were expected for the rest of the week, with temperatures expected to be in the low 20s.