A state of emergency was declared in Australia's most populated region on Thursday, as a record heat wave fanned unprecedented bushfires.About 100 fires have been burning for weeks in drought-plagued New South Wales (NSW) with half of them uncontained, including a "mega-blaze" ringing Sydney, covering Australia's biggest city in a haze of toxic smoke.There were 2 000 firefighters battling the blazes Thursday, with the support of small US and Canadian teams, as well as Australia Defence Force personnel.WATCH | Hundreds of Australia homes destroyed in 'mega fire'NSW Premier Gladys Berejiklian said the seven-day state of emergency, the second declared in the state since the bushfire season began early in September, was due to "catastrophic weather conditions".Temperatures are expected to near 50°C in South Australia and peak at 45°C in the western suburbs of Sydney, while turbulent winds of up to 100 kms are expected to fan bushfires burning ever-closer to the city.The country experienced its hottest day on record Tuesday, with the average nationwide temperatures reaching 40.9°C, which is expected to be surpassed as an intensifying heat wave spreads across the country.'Strike teams' on standbyNew South Wales Rural Fire Service commissioner Shane Fitzsimmons said five 100-person "strike teams" were on standby to deploy to the most dangerous fires given the "enormity of some of these fire complexities and the severity of the forecast weather conditions"."The worst of the fire weather conditions, the extreme fire danger ratings we are expecting today, are centred around the greater Sydney environment," he added.At Buxton, about 100kms southwest of Sydney, longtime resident Paul Collins said a nearby bushfire that had destroyed properties was "much worse" than in past years.READ | Sydney smoke crisis 'longest on record', authorities say"It's spread faster with the wind, and the bush and the ground is just so dry," he told AFP, blaming climate change and the drought for the worsening fires."It's just a horrendous situation, really."On Thursday climate protesters marched on Prime Minister Scott Morrison's official residence in Sydney to demand curbs on greenhouse gas emissions and highlight his absence on an overseas holiday as large parts of the country burn.'Dangerous and disastrous' heatThe extreme weather is causing major health concerns, with leading doctors this week labelling the smoke haze that has shrouded Sydney for weeks a "public health emergency".New South Wales health officials are urging vulnerable people - particularly the elderly and those suffering chronic conditions - to stay indoors amid worries the scorching heat combined with toxic bushfire smoke could cause "severe illness, hospital admissions and even death".Bureau of Meteorology forecaster Dean Narramore said the "dangerous and disastrous" heat wave was toppling dozens of "extraordinary" records across the country.Scientists say the blazes have come earlier and with more intensity than usual due to global warming and a prolonged drought that has left the land tinder dry and many towns running out of water.