A total fire ban is in place across Sydney and the Hunter region as NSW residents face temperatures soaring into the mid-40s.
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.
"Hot and windy conditions are forecast over the Sydney area," the Bureau of Meteorology said in its fire weather warning.
"A front will bring cooler conditions to coastal areas during the afternoon and through the west in the evening."
The NSW Rural Fire Service is warning residents to prepare their bushfire plans. If people are considering leaving their homes they should "leave early", the RFS said.
Temperatures are forecast to hit 40C in Sydney's CBD. In the western suburbs, the mercury will soar to 45C in Penrith.
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."
Sydney Water is encouraging residents to keep well hydrated throughout the day.
"Remember to (also) provide water for your pets and water your garden either early in the morning or in the early evening so that your plants gain the benefits," spokesman Peter Hadfield said in a statement.
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.
A total fire ban means no fires can be lit in the open and all fire permits are suspended.