At least 100 homes are feared to have been destroyed by intense wildfires which tore across southeastern Australia in horror wind conditions Thursday, as firefighters warned of "substantial" property damage.

Seven major blazes were burning across the state of New South Wales, fanned by high, erratic winds in unseasonably warm 34 Celsius (93 Fahrenheit) weather, with infernos at Springwood and Lithgow in the Blue Mountains west of Sydney sending thick plumes of smoke and ash across the city.

"If we get through with less than 100 homes destroyed today, we have been lucky," NSW Rural Fire Service Commissioner Shane Fitzsimmons tweeted of the blazes. No injuries or fatalities have been reported.

Springwood man Joe Moore told Sky News his home had been razed while an estimated 40 other houses had been lost in the village, though that figure could not be confirmed by officials.

"It's about as bad a situation as we could ever have hoped for," he said.

NSW Emergency Services Minister Mike Gallacher said the unconfirmed reports "from both police and firefighters in the field around Springwood is we're seeing substantial losses to property, homes".

"The footage we're seeing as well as the reports from the fire ground is that (the situation) is not good," he said.

The NSW Rural Fire Service said it was bracing for "erratic and dangerous conditions for the next few hours" as a southerly wind came in, with flames descending the mountains and jumping the Nepean River on the outskirts of Sydney to start a fire near Penrith.

"Reports of homes being lost across several fire grounds," said Rural Fire Service deputy commissioner Rob Rogers. "Still have several hours of unpredictable fire activity (ahead)."

Gallacher said fire crews were reporting some of the worst conditions seen in years, with quick-moving blazes of "great ferocity" and smoke so thick it was showing up on weather radars.

Strong winds forced the grounding of water-bombing firefighting aircraft and residents of the Blue Mountains were urged to stay off the roads to clear the way for emergency vehicles.

Wildfires are common in Australia's summer months between December and February, and authorities are expecting a bad season this year due to low rainfall in the winter and forecasts of hot, dry weather ahead.

A devastating firestorm ripped through southern Victoria in 2009, razing thousands of homes and killing 173 people.