Sydney swelters through 40C temperatures - but a cool change is coming

NSW paramedics are bracing for a busy day as the state swelters through scorching summer conditions but a cool change is set to bring some much needed relief to Sydneysiders.

Temperatures are tipped to exceed 40 degrees Celsius on Wednesday in several parts of NSW but a southerly change moving along the state's south coast will bring relief during the day as it tracks north.

Authorities have warned residents, particularly those aged 60 and over, to stay out of direct sunlight, drink plenty of water and never leave children or pets in cars.

The NSW RFS has issued total fire bans for the Greater Sydney Region, The Hunter Region and North Western NSW, with the three areas facing severe fire danger ratings.

A cool change is expected to be a welcome relief for Sydneysiders. Source: BOM

"It's pretty hot and dry right around different parts of the state, but that's where the concentration of the worst weather is today," RFS Commissioner Shane Fitzsimmons told the Nine Network on Wednesday.

"We have got 30 fires that we still haven't got contained; we are dealing with more than 80 fires around NSW at the moment."

However, some of those concerns may be eased, with temperatures expected to drop at least 10 degrees on Wednesday afternoon.

The southerly change is heading north and should hit Sydney's coast between 1pm and 2pm before heading to the city's south.

Canberra and Coffs Harbour are expected to have minimums of 21, while Tamworth, Newcastle and Wollongong will have overnight lows of around 23.

While the city is tipped to remain around 35C for much of the working day, showers and southerly winds are tipped to see temperatures plummet to 26C by 5pm.

Temperature will continue to drop into the night and are expected to stay down for much of Thursday and Friday, with showers expected for both days.