Bushfires that destroyed six homes and up to 100 cars across New South Wales have eased slightly, with lower temperatures bringing some relief to firefighters.

However, dozens of residents spent the night at an evacuation centre as fires at Port Stephens, north of Newcastle, threatened properties.

Five homes were reportedly destroyed there while another may have been lost in a bushfire at Wisemans Ferry on the Hawkesbury River.

Port Stephens Mayor Bruce MacKenzie said the homes should not have been lost.

"We've got a situation today where no one's allowed to burn off," he said.

"The RFS (Rural Fire Service), politicians and the greenies have got a lot to bloody answer for because if there's five houses lost it could have been prevented."

Firefighters are working throughout the night to control blazes at Salt Ash and Fingal Bay in Port Stephens, fearing a wind change that could turn the fires into the path of properties.

Get the latest information from the Rural Fire Service website.

Residents at Salt Ash are being warned of continuing ember showers, which could spark blazes ahead of the fire front.

Near the Heatherbrae area, firefighters are struggling to contain a 100-hectare fire that is burning towards properties on Tomago Road between Masonite Road and Graham Drive in Tomago.

Cars are being stopped along Lemon Tree Passage Road because of smoke from a bushfire at Salt Ash. ( ABC Newcastle Reporter: Clare Mortensen )

Rural Fire Service spokesman Joel Kursawe said a cool change was expected to bring some relief, but there are fears of a strong wind change.

"When this change comes through it has the potential to turn the flanks or sides of these fires into a fire front," he said.

At Hawkesbury, about 100 firefighters were working into the early hours of Monday morning to contain a large fire that was moving north-east.

The fire has burnt more than 180 hectares of bushland.

Grass fire incinerates cars

On Sunday, up to 100 cars were incinerated in a car park at Sydney's Olympic Park Aquatic Centre as hundreds of people enjoyed an afternoon swim.

Fire and Rescue NSW superintendent Ian Krimmer said the inferno was started by a grass fire, fanned by strong winds.

About 500 people were evacuated from the swimming centre.

Sorry, this video has expired Firefighters battle the car park blaze at the Olympic Park Aquatic Centre.

The blaze was later contained and there were no reports of injuries.

One witness told ABC local radio that the fire spread very quickly.

"You could see flames underneath some of the cars and the next thing I knew was that within three or four minutes this black smoke was blowing right across the car park and you could hear popping sounds," the witness said.

"So obviously, each of those pops was another vehicle going up."

Difficult conditions for firefighters

On Sunday, a large part of the state was experiencing severe to extreme fire conditions as temperatures rose well into the 30s and hot winds picked up.

New South Wales deputy fire commissioner Rob Rogers said conditions were extremely difficult for firefighters.

"The conditions that we have, certainly up in that Hunter area at the moment, [are] 34 degrees, 14 per cent humidity, north-westerly winds at 57 gusting to 82 kilometres an hour so that's very, very difficult and very dangerous."