Poor air quality has been forecast for parts of the state over the coming days as ozone levels rise with the hot weather, NSW Health says.
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The temperature is expected to hit 33 degrees on Wednesday and Thursday at Albion Park before peaking at 36 degrees on Friday. Wollongong is forecast to nudge 30 degrees over the next two days and 31 degrees on Friday.
The Bureau of Meteorology forecast daytime temperatures up to 12 degrees above average and 10 degrees higher than usual at night until Friday.
Parts of the state had almost reached 40 degrees before 9am on Wednesday.
Authorities are reminding people to take extra care during the extreme heat by staying indoors, minimising physical activity and keeping hydrated.
"We know that conditions like these can cause severe illness, hospital admission and even deaths," NSW Health's Dr Richard Broome said.
People over 75 and those with chronic medical conditions, as well as those living alone, are particularly vulnerable.
Air quality is also expected to worsen because of the heat, with high ozone levels forecast over the coming days.
Dr Broome says ozone can irritate the lungs and is advising people with asthma to keep their relieving medication on hand.
"Ozone levels are higher outdoors than indoors, and generally highest in the afternoon and early evening, so limiting time outdoors during the heat of the day and in the evening helps people to not only keep cool but to limit their exposure to ozone pollution" he said.
By 10.30am on Wednesday, the mercury had hit 28 degrees in Albion Park while Moss Vale and Nowra were both over 30 degrees.
Areas in the central west had already recorded temperatures above 35C by 9am.
Cobar Airport and Noona passed 38C, while Broken Hill, Ivanhoe and Bourke reached 37C.
The small town of Tarcoola in South Australia hit an eye-watering 49 degrees on Tuesday.
- with AAP