Parts of NSW were already nudging 40C before 9am, as heatwave conditions sweep across the state.
The majority of NSW is forecast to exceed 41C from Wednesday to Friday - the first time since 1939 most of the state has endured such high temperatures for five consecutive days.
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 Richard Broome said.
Beachgoers cool off at Bronte Beach in Sydney.
AAP
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 across Sydney.
Dr Broome says ozone can irritate the lungs and is advising people with asthma to keep their relieving medication on hand.
Sydneysiders are being urged to be careful, due increased ozone levels caused by the heat.
AAP
"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 9am on Wednesday, areas in NSW's central west had already recorded temperatures above 35C.
Cobar Airport and Noona passed 38C, while Broken Hill, Ivanhoe and Bourke reached 37C.
Most of the Riverina was sitting in the mid-30s, as was the central west slopes and plains.
In Sydney, Penrith tipped past 28C just before 9am, while Camden, Campelltown and Badgerys Creek weren't far behind at above 27C.
Observatory Hill, in the city's CBD, was nudging 27C.
