Parts of Australia's southeast are sweltering through extreme temperatures and bushfires, and it will be a few days yet before a cool reprieve.

Sydney set its highest-ever 9am temperature today, with the city reaching 36.5C, beating the previous record of 35C set in December 1955.

In Queensland, temperatures reached the mid-20s before sunrise.

Sydney can prepare to finally cool down today with temperatures dropping dramatically across the city after earlier highs of 36.5C. (Adam Bovino/9NEWS)

Sydney's sweltering morning came after a sticky summer night, with temperatures not dropping below 28C in the CBD.

At 6am the mercury was sitting at 31.2C in the city.

In Sydney's west temperatures dropped to the low-20s overnight, but had reached 42C at midday.

The cool change follows an evening of sweltering heat across the state. (Adam Bovino/9NEWS)

"I woke up and I just wanted to be in the water," a Bondi swimmer told 9NEWS.

A southerly change began to sweep through the some suburbs at midday but did not reach the city's west before 3pm.

Sydney Airport experienced a ten-degree drop in about three minutes, according to Weatherzone.

"It looks like it could be one of the warmest runs of higher than average minimum temperatures on record," Jacob Cronje from Weatherzone told 9NEWS.

Sydney city can expect the mercury to be sitting at 25C by 5pm. (Adam Bovino/9NEWS)

Milder conditions will give residents some relief tomorrow, with a maximum of 25C in the city and 27C in the west, but the temperature is due to climb back up again on Friday.

Similar conditions are expected next week, with experts warning no lasting relief is in sight any time soon.

With AAP.