Sydney residents woke to find their city under a blanket of smoke on Thursday and it is expected to remain at least until Friday.The smoke is due to around 50 bushfires still burning in the Australian state of New South Wales, affecting the north and central coast and Illawarra as well as Sydney.A poor air quality forecast alert was issued in the early hours of the morning and the Department of Primary Industries and Environment's air quality index showed hazardous fine particle readings in central Sydney by 9am.Image:The Sydney Harbour Bridge was almost hidden by the smokeMore than 600 homes have been destroyed by bushfires in New South Wales during the current fire season, 503 of them in the past two weeks, according to the NSW Rural Fire Service.Six people have been killed.But the service estimated that more than 7,000 threatened buildings had been saved, either by firefighters - who were supported by colleagues from other states and from New Zealand - or members of the public.The fire danger has also spread to other states, including South Australia and Victoria.In South Australia, at least 50 fires were burning on Thursday, with officials lifting the fire danger warning to "catastrophic".This means that, should a fire start in conditions where temperatures were expected to soar past 42C (107.6F), firefighters would not be able to control it.On Wednesday, power was cut to more than 12,000 homes and businesses as some of the fires approached electricity lines.A fire in Yorketown, about 56 miles west of Adelaide, reportedly destroyed 11 properties and left 33 people with minor fire-related injuries.Image:Dozens of fires are still burning in New South Wales, with many more in other statesIn Victoria, communities in the north and north-west were told to leave their homes as parts of the state were issued their first Code Red - the highest danger level - since 2010.The state capital Melbourne was facing its hottest November day in almost 20 years on Thursday, with forecasts predicting a top of 39C before an afternoon thunderstorm was expected to bring the temperature down 20C in half an hour.Victoria Country Fire Authority chief officer Steve Warrington said: "Homes are not built to withstand the types of fires we may see on a Code Red day and you don't want to be caught travelling through areas on fire at the last minute if you wait and see."There was some relief in Queensland, however, with a state of emergency being lifted from 36 of 42 local government areas.Acting Queensland Fire and Emergency Services Commissioner Mike Wassing said "significant blazes" continue to burn in the six remaining areas where the risk "remains high".