Fires expected as NSW temp records fall

More new September temperature records in NSW are expected to coincide with another difficult day of firefighting.

A total fire ban has been declared for Greater Sydney, the Illawarra and most of the state's northeast on Sunday.

High temperatures, low humidity and wind are expected to lead to further outbreaks of fire.

"Another difficult day ahead," the NSW Rural Fire Service tweeted on Saturday evening.

More than 90 fires started on Saturday including one midway between Sydney and Canberra that caused the closure of the Hume Highway.

When the main arterial between Melbourne, Canberra and Sydney reopened, traffic trailed back seven kilometres in one direction.

Sydney is forecast to reach 33C on Sunday while some parts of the state's north may top 40C.

No place in NSW had recorded 40C on a September day before Wilcannia, Bourke, Gumbalie and White Cliffs all cracked the mark on Saturday.

The Bureau of Meteorology estimates roughly 40 per cent of weather stations had their hottest September day on Saturday.

While Sydney didn't break a record, Saturday's top temperature of 32.2C was almost 12C above average.

"For this time of year, it's been very unusual," bureau senior forecaster Jake Phillips told AAP.

Temperatures will ease after the weekend before heating up again mid-week.

A weak rain band is forecast to cross NSW on Thursday or Friday.

"I'm sure the rain will be useful - particularly to farmers - but it doesn't look like it is going to be enough," Mr Phillips said.