People in NSW are set to sweat through the last days of the year, with an extreme heatwave affecting parts of the state.
Penrith is expected to hit a 42C maximum on Thursday, the day after parts of Sydney's west endured 37 degrees.
BOM duty forecaster Neale Fraser said it would be a few degrees cooler in the CBD, where a sea breeze expected around 1pm is expected to cap temperatures at 37C.
But it could be hotter elsewhere across the city, he said.
"40C (is forecast) for most of Sydney and then right on the coast the sea breeze might keep it down to low to mid 30s," he told AAP.
An extreme heatwave on and east of the ranges and the mid north coast is expected to move north over the weekend.
A very high fire danger is forecast for the state's capital on Thursday.
The heatwave has prompted NSW Health to issue warnings, especially in the north where a slow moving cool change is not expected to hit until Monday.
People are urged to stay hydrated, avoid alcohol and limit physical activity.
Surf Life Saving NSW is also asking people to apply sunscreen and seek attention from lifeguards if they experience signs of heat stress, such as excessive sweating, rapid heart rate and nausea.
