Gale force winds have fanned two of Australia's massive bushfires into a feared "mega blaze", with authorities warning of worse weather to come. It had been feared for days that fire would spill over the New South Wales-Victoria border in the Snowy Mountains. Forecasts are for more heat, strong winds and dry lightning. In South Australia, firefighters also battled infernos on Kangaroo Island. In parts of both states, residents were told to leave their homes. Meanwhile, tens of thousands of people across Australia took part in climate change protests on Friday. In cities including Sydney, Melbourne and Canberra, demonstrators turned out to press the government of Prime Minister Scott Morrison to make a quick transition away from fossil fuels. Across many parts of the country deadly forest fires that have raged for weeks are threatening to advance again as temperatures soar. The winds mean fires could spread quickly and unpredictably this weekend. The mega blaze south of the Snowy Mountains came after two fires at Dunns Road and East Ournie Creek joined up, following another massive fire merger nearby earlier in the week. An area totalling nearly 600,000 hectares (1.5m acres) - about four times the size of Greater London - is now ablaze. New South Wales (NSW) Rural Fire Service spokesman Anthony Clark said a "finger" of the East Ournie Creek blaze had collided with the fire at Dunns Road on Friday evening. Mr Clark said a number of small fires started by lightning strikes had merged and grown, the Sydney Morning Herald reports. "It provides a challenge for firefighters as, when they merge, it increases the size and opens up more uncontained perimeter." Fires in NSW have destroyed about 1,000 homes since the New Year and more than 150 bushfires are burning there. But the danger is equally great further south in Victoria. Victoria's Country Fire Authority issued several emergency warnings on Friday, telling people to evacuate before it became too dangerous. Making the task harder for fire crews, aircraft were unable to operate overnight in the dark.