Stay safe during heat: NSW authorities

NSW residents are being told to prepare for a scorcher of a weekend, with warnings to drink plenty of water and to take care in the surf, and prepare properties for the increased bushfire danger.

The Bureau of Meteorology has warned of severe heatwave conditions for much of the state, with the mercury expected to peak on Sunday.

The Rural Fire Service at lunchtime on Saturday said there were 10 bush and grass fires burning across NSW with four of them - in Wollondilly, Tamworth, Cobar and Wollongong - uncontained.

The RFS in a tweet said no homes were under threat and 70 firefighters would work on the fires on Saturday ahead of worsening conditions on Sunday.

Parts of the state are tipped to reach 45C on Saturday, including Menindee in the far west and Ivanhoe in the state's centre.

Most of the state will swelter through temperatures above 40C on Sunday, including Bulahdelah on the mid-north coast which is set to reach 45C.

In Sydney temperatures are forecast to peak at 38C in the city on Sunday, while the mercury could hit 45C in Penrith.

"A NSW Health study published just last month found extreme heatwaves lead to a more than 10 per cent increase in both deaths and ambulance callouts," Dr Ben Scalley, Director of Environmental Health, said in a statement on Friday.

"They put a lot of strain on the body and can cause dehydration, heat exhaustion and heat stroke."

NSW Police deputy commissioner Catherine Burn said the state's heatwave plan had been activated to ensure a coordinated response from all emergency services.

"We know over the next two or three days we're going to experience severe to extreme heat conditions throughout NSW, particularly in western Sydney, western NSW and the mid-north coast," she told reporters in Sydney on Saturday.

Ms Burn also reminded people that it is an offence for children or pets to be left alone in a vehicle.

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

"We've already seen some incidents in the last five weeks, one unfortunately where a dog died after being left in a car and where we arrested and charged a man allegedly for leaving a child in the car in hot conditions."

Police have urged livestock and pet owners to ensure clean water is always available, shading is provided where possible and no animals are left in confined, unventilated areas.

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.

NSW Health urged people to drink plenty of water, but say they are prepared for a rise in emergencies during the heatwave.