New South Wales's premier has lashed out at "scumbag" looters who reportedly raided evacuated homes across the south-eastern Australian state, while fire-fighters continue to struggle with numerous bushfires that are at risk of merging into a massive blaze.

More than 200 homes have been destroyed in fires that have flared up across NSW, whose capital is Sydney, since last week. The outbreak has prompted authorities to declare a state of emergency.

As hundreds of people were evacuated, police said they started receiving reports that looters were targeting abandoned homes.

"I'm just appalled that at these critical times, when people have been evacuated from homes or whether people have left homes because of fire dangers, that other scumbags in the community would front up and seek to rob them," NSW premier Barry O'Farrell said.

"Communities are angry but importantly I know that police are determined to track these people down."

Emergency services said that at least 58 fires were still burning, including 14 that remained out of control.

The worst affected area was between the town of Lithgow, west of Sydney, and the Blue Mountains - where authorities feared a massive blaze which has already burned nearly 40,000 hectares (99,000 acres) could join with others to create a huge fire stretching over 300km.

"I don't think I've ever used the word mega-fire," said New South Wales Rural Fire Service Commissioner Shane Fitzsimmons.

"But the reality is that the modelling indicates that there's every likelihood that in the forecast weather conditions that these two fires, particularly up in the back end of the mountains, will merge at some point."

Localised rainfalls are forecast to bring some relief in the next 24 hours but conditions should then worsen, with high temperatures and strong winds reaching between 80km/h and 90km/h expected later this week.

Four children have been charged with fire-related offences.

An 11-year-old boy was charged with starting two fires near Newcastle, while two girls aged 12 and 13 were charged with deliberately lighting a grass fire west of Sydney and a 14-year-old boy was charged over a fire at Rutherford.

"It's awful to think, despite the universal and quite right public condemnation of the deliberate lighting of fires, that we can have young people so reckless and so stupid participating in these sorts of activities," said Fitzsimmons.

"It's one thing to be an adult criminal in setting fire, it's another thing, and it saddens me, that young people could actually not see the consequences of this sort of reckless behaviour."