The mercury struck a painful 47.3 degrees just before 3.30pm in Penrith, the highest reading since February 11, 2017 when a temperature of 46.9 degrees was recorded.

It fell just short of a Sydney metropolitan region record, with the Bureau of Meteorology briefly announcing it as the hottest Sydney day before retracting that for a reading of 47.8 degrees recorded at a Richmond station in 1939.

Just after 1pm the CBD recorded its hottest temperature in five years, as the heat reached 43.4 degrees. The hottest day in the city on record occurred on January 18, 2013, when it reached 45.8 degrees.

NSW Health activated its heatwave plan on Friday afternoon, ensuring resources were appropriate for the demand.

Ben Scalley from NSW Health said it was difficult to monitor heat-related hospital admissions over the weekend because heat affects those with pre-existing conditions.

"What we've actually found is that people who come in don't just come in with obvious issues like heatstroke, it's actually any existing medical conditions can be exacerbated by it," he said.

During heatwaves there is about a 10 per cent increase in calls for ambulances, as well as a 3-4 per cent increase in emergency calls, and 10 per cent increase in deaths, Dr Scalley said.

As Sydney's beaches filled up with those desperate to cool down, Surf Life Saving NSW geared up for a crucial weekend of patrol.

One drowning was confirmed on Sunday, bringing the total number of drownings to 21 for this Surf Life Saving season - just 10 below last year's entire season.

The 25-year-old man slipped and fell into water while walking on rocks at Little Beecroft Head, near Currarong around 9.30am on Sunday.

Two of the men he was with attempted to rescue him, but the three were pulled from the water by Marine Rescue Shoalhaven, with the 25-year-old unable to be revived.

Surf Life Saving NSW reported that by Saturday afternoon, at least 32 significant rescues had been carried out between Sydney and Illawarra over the weekend.

Significant rescues are those that would result in injury or worse if surf lifesavers and lifeguards did not retrieve the person. However these numbers are preliminary, and will increase as busy clubs get the chance to report their rescues after a chaotic weekend, Surf Life Saving said.

That number included 14 rescues on the central coast, five on the north coast and seven in Sydney, including four at Bondi Beach.

NSW police confirmed a 33-year-old man was in a critical condition in Wollongong Hospital on Sunday afternoon, after he was rescued from the water by members of the public at Port Kembla.

This followed another incident on Saturday, when a 38-year-old man was pulled unconscious from the water at Sandon Point Beach before being taken to Wollongong Hospital in a critical condition.

The NSW Office of Environment and Heritage issued an air quality forecast of poor for Sunday, with ozone levels higher than normal.

By the afternoon readings on the air quality index were slightly better than expected.

Dr Scalley said the poor ozone level over the weekend meant it was crucial that those with respiratory issues, such as asthma, took extra care.

As Sydneysiders watched in horror and awe, they turned to social media to upload photos of their dashboard temperature readings.

Duty forecaster at the Bureau of Meteorology Andrew Haigh reminded people that surrounds have a big impact on measuring temperature.

"The local conditions do affect the temperature that you're reading, so if you're driving around a hot road, the car dashboard might be hotter than surroundings away from the road," Mr Haigh said.

"All our weather stations are standardised to make the conditions in which measurements are taken the same," he said.

He said the biggest mistake was taking a measurement while standing in direct sunlight.

The heat also left thousands of people across the state temporarily without power on Sunday evening, according to electricity provider Ausgrid.

Just after 9pm on Sunday, about 1400 properties in Punchbowl and Bankstown were without power, 70 in Riverwood, 70 across Hurlstone Park, Dulwich Hill and Ashfield, about 1500 across Narara, Somersby and Lisarow, 700 in Chatswood, and 500 in Lane Cove North and Lane Cove.

Most of these properties were expected to have their power restored between 10-11.30pm on Sunday.

Temperatures stayed high as the day drew to a close, with a reading of 33.2 degrees in the CBD at 5.20pm, while Penrith remained at a scorching 45.1 degrees.

The heat will linger, with Monday's forecast high 33 degrees in the city and 40 degrees in the west, then a slight drop to 30 degrees in the city and 37 in Penrith on Tuesday.

The city looks set to trail off into the 20s come Wednesday, although, except for Wednesday, Penrith's current top temperatures are all forecast in the 30s for the coming week.