Australian firefighters were racing against the clock to bring a series of blazes under control Thursday before a forecast spike in temperatures brings the risk of more infernos.

Fires have been raging across Australia for nearly a week and while many have been contained, 126 are still burning and at least 15 remain out of control in the country's most populous state, New South Wales.

A cooler weather front that brought some relief on Wednesday continued in many parts Thursday, but temperatures are set to soar once again to well over 40 degrees Celsius (104 Fahrenheit) on Friday, piling pressure on firefighters.

NSW Rural Fire Service commissioner Shane Fitzsimmons said crews were working flat out to do all they could before the heat returned.It's about focusing on getting as much contained and consolidated as we can ahead of a return to hotter and dryer conditions dominating much of NSW over the coming days," he told ABC television.

"We're looking at temperatures across much of NSW into low-to-mid 40s and extending into the high 40s on Saturday.The only reprieve, if you can call it that, is that we are not expecting significant wind strengths to build.

"But it is almost academic. With such hot, dry and dominant (weather) movement from the northwest, even a moderate breeze is going to beproblematic and risky for communities and firefighters over the weekend."

The blazes have burned more than 350,000 hectares (865,000 acres) of land in New South Wales alone, and while more than 100 homes were razed in Tasmania state last weekend, only a handful have been destroyed around the country since then.No deaths have been reported.

The biggest impact has been on farmers, with vast amounts of pasture, crops and animal feed lost, as well as thousands of head of stock and agricultural infrastructure such as sheds and outbuildings.

One of the worst-hit areas is Yass Shire west of Canberra where a fire has so far burnt out 16,000 hectares and killed 10,000 sheep.As well as New South Wales, fires continue to burn in the states of Victoria, Tasmania and Queensland.

Wildfires are a fact of life in arid Australia, where 173 people died in the 2009 Black Saturday firestorm, the nation's worst natural disaster of modern times.Most are ignited naturally, but in Sydney's west three teenage boys were charged with deliberately lighting a fire on Tuesday and on Wednesday a man was charged after sparks from his angle grinder caused a blaze.

Police said he was using the power tool on the verandah of his property near Mudgee, northwest of Sydney, when sparks ignited nearby grass. So far the fire has destroyed 140 hectares of farmland and was still burning.