More than 600 firefighters are battling 68 fires across NSW as the state seeks cool relief from a spring heatwave.

A total fire ban is in force for the Greater Sydney, Hunter and Upper Central West regions due to the extreme danger caused by the record-breaking September heat.

The Rural Fire Service said 17 blazes were yet to be contained as of 9am on Wednesday.

NSW Premier Chris Minns said he was concerned about the temperatures given the state had only just entered the spring season.

“It’s September and we’re already experiencing four days in a row of temperatures above 30 degrees with high winds,” he told reporters.

“We are concerned about summer.”

The unseasonably hot and dry conditions are being felt across southern Australia, with temperatures peaking at 8-16C above average across much of South Australia, NSW and Victoria.

Sydney is set to hit a maximum 35C on Wednesday before a cool southerly change starts to move up the south coast in the afternoon. 

Mr Minns urged NSW residents to make sure they were prepared for the upcoming fire season. 

“It’s a good warning to the communities of NSW to have a fire preparation plan in place for the summer that’s coming up,” he told ABC News.

“It’s important for every community, we want to make sure that lives and property are safe during the summer period.”

The Australian Bureau of Meteorology declared an El Nino event on Tuesday afternoon, warning of hotter and drier conditions that could lead to severe heat waves, droughts and bushfires, particularly in the east. 

By Maeve Bannister in Sydney

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