More than 50 bushfires were burning in New South Wales on Saturday afternoon, the state's Rural Fire Service said, although the vast majority were considered "under control".

A blaze north of Sydney had destroyed six houses, national broadcaster ABC said, while a small number of homes were lost in a bushfire on the state's mid-north coast.

One bushfire had burned through more than 9,000 hectares (20,000 acres) in the Goulburn River National Park.

Bureau of Meteorology forecaster Dean Narramore said hot and dry winds were fanning temperatures across the state nearing 40C.

"This is a dangerous few hours," he said.

"Unfortunately this heat has been associated with hot and gusty winds as well, and that's what really driving these dangerous and extreme fire dangers."

Bushfires are a common occurrence in Australia's summer months, and it is not unheard for dozens of blazes to burn through sparsely populated areas on hot and windy days.