Fears have been raised that a ‘mega-fire’ could develop if a major bushfire in southeast Australia merges with other blazes.

Firefighters have been battling fires that flared in high winds and heat across New South Wales last week which destroyed 200 homes and left many others damaged.

While many blazes have been contained, 58 were still alight and 14 of them out of control, enveloping Sydney in a thick white smoke haze that prompted warnings for people to stay indoors and avoid exercising, Sky News reported.

The main area of concern was near the town of Lithgow west of Sydney where a huge fire that has already burned nearly 99,000 acres was threatening the communities of Bilpin, Bell, Clarence and Dargan.

Officials fear intensifying heat and winds this week could push it into another blaze at nearby Mount Victoria in the Blue Mountains and then move towards the populated areas of Katoomba and Leura.

“I don’t think I’ve ever used the word mega-fire,” New South Wales rural fire service commissioner Shane Fitzsimmons was quoted as saying.

“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.”

New South Wales declared a state of emergency on Sunday amid the worst fire disaster in the state for almost 50 years. This gives fire crews the power to forcibly evacuate people, with penalties for refusing.

Emergency services minister Mike Gallacher said every possible resource was being used, including firefighters being drafted in from interstate and the possibility that the military could also be deployed.