Sydney swelters as temps hit 45C by midday and record melts in city's west

Sydney has endured one of its hottest days in recorded history, with the preliminary high in Penrith reaching 47.3C - a record for that station.

That's slightly higher than the at 47.0C record at a weather station in Sydney's west last February, the Bureau of Meteorology says.

Initial reports from the bureau suggested it was a new record for the city, but it later clarified in a tweet that an old Richmond weather station recorded 47.8C in 1939.

Regardless, it was horrendously hot and thousands headed to Sydney's beaches on Sunday with Bondi packed with bathers soaking up the sun while finding relief in the cool waters of the Pacific Ocean.

Hundreds of parasols were spotted scattered across the sand with many paying attention to NSW Ambulance warnings over the extreme heat.

The temperature was so unbearable at one Woolworths, the supermarket giant handed out free bottles of iced water.

"Free cold water!! Cause (sic) it's way too bloody hot!!" a sign read accompanying a cool box filled with ice.

A severe fire danger rating has been issued for the greater Sydney region on Sunday while much of the rest of the state has a "very high" rating.

Total fire bans were implemented and remain in place for Sydney and the Hunter region.

The NSW Rural Fire Service warned residents to prepare their bushfire plans. People considering leaving their homes they should "leave early", the RFS said.

The CBD's temperatures soared throughout the day with a recording of 43.4C at Sydney's Observatory Hill just after 1pm.

The Bureau of Meteorology said cooler conditions were expected to reach coastal areas during the afternoon and Sydney's west in the evening.

NSW Health warned people to drink plenty of water and limit their time outdoors because of a rise in ozone pollution as a result of the hot, still weather.

Sydney was forecast to have poor air quality on Sunday which can especially affect people with asthma and chronic obstructive pulmonary disease.

"Ozone levels are higher outdoors than indoors, so limiting time outside during the heat of the day and in the evening would help people to keep cool and to limit their exposure to ozone pollution," Environmental health director Dr Ben Scalley said in a statement on Saturday.

Dr Scalley also warned of the dangers of a heatwave which put strain on the body, can cause dehydration, heat exhaustion and heat stroke.

NSW Police Deputy Commissioner Catherine Burn on Saturday revealed the state's heatwave plan had been activated to ensure a coordinated response from emergency services.

"We know over the next two or three days we're going to experience severe to extreme heat conditions throughout NSW," she said before reminding people it was an offence to leave children or pets in vehicles.

"Cars become a furnace very, very quickly in this type of heat."

Surf Life Saving NSW has implored people heading to the beach to take care given there have been 10 drownings since the beginning of December.

A 48-year-old is fighting for his life after he was found face down in the water at Sandon Point Beach on Saturday.

In a separate incident, a 35-year-old man was taken to hospital in a stable condition after being pulled semi-conscious from the Hawkesbury River at Lower Portland.

While Sydney swelters through their record-breaking day, further west several areas of NSW, as well as Canberra, are subject to a thunderstorm warning with winds of up to 90km/h predicted.