Several bushfires have combined to form what is being called a 'mega fire' in a national park forest north of Australia's biggest city, Sydney. Firefighters warned late Friday that they are struggling to contain the blaze, which continues to burn across at least 300,000 hectares (741,000 acres) of land and has spewed smoke and haze over the city.

Outdoor sports have been canceled and health authorities in the state of New South Wales (NSW) reported a spike in respiratory illness as smoke from the fires blanketed Sydney. Public health experts are warning people in fire-affected areas to prepare for a prolonged period of poor air quality.

The NSW Rural Fire Service said firefighters can do little more than try to get residents out of the way, protect property and wait for dry and windy conditions to subside before they can begin containing the fires. An official warned that, without "flooding rain," the mega fire could burn for weeks. A 3-month-long drought has turned eastern Australia into a tinderbox.

Fires have threatened properties, including in Werombi, south-west of Sydney. The Rural Fire Service said Friday that more than 680 homes have been destroyed and 250 others have been damaged by bushfires in the state since early October.

On Saturday, an estimated 2,800 firefighters were struggling to extinguish more than 100 wildfires across NSW. The closest wildfire to Sydney is burning 75 kilometers (46 miles) northwest of the city center.

Smoke from the fires can be seen from space. Wildfires are common in Australia's summer, which begins in December. This year, however, the fires started in October, driven by persistent high temperatures and dry winds. In the coming days, temperatures in NSW are forecast to crack 40 degrees Celsius (104 Fahrenheit), and meteorologists say no meaningful rainfall is expected until late January.