(Recasts on fires converging)
By Colin Packham and Byron Kaye
SYDNEY, Dec 6 (Reuters) - Bushfires fanned by winds combined into a single giant blaze north of Sydney late on Friday, blanketing Australia's biggest city in hazardous smoke, causing weekend sports games to be cancelled and prompting calls for outdoor workers to stay home.
Wildfire has killed at least four people and destroyed more than 680 homes across eastern Australia since the start of November, months earlier than the usual summer bushfire season, with the authorities blaming an extended drought, strong wind and suspected arson.
Smoke and flying ash has lingered over Sydney for most of the past week, turning the daytime sky orange, obscuring visibility and prompting commuters to wear breathing masks. Satellite images spread on social media showed the smoke spreading across the Tasman Sea to New Zealand, 2,000 kilometres (1,240 miles) away.
"The massive #NSW fires are in some cases just too big to put out at the moment," wrote the Australian Bureau of Meteorology in a post on its official Twitter account.
Sydney is the capital city in New South Wales state.
The New South Wales Rural Fire Service (RFS) said in a tweet that "a number of fires in the Hawkesbury, Hunter and Central Coast areas have now joined". The tweet included a map showing 10 separate fires connecting about 50 kilometres north of metropolitan Sydney.
The total area burnt by those fires was around 335,000 hectares, or 830,000 acres, the RFS added.
RFS Commissioner Shane Fitzsimmons said authorities were particularly concerned the fires may spread east.
"They have the potential or are expected to spread further east, which unfortunately is getting into more populated areas, villages, communities, isolated rural areas, and other farming practices and businesses throughout the region," Fitzsimmons told reporters in Sydney.
About 500 homes in coastal communities around 350 kilometres south of Sydney had lost electricity to fire and repair workers were unable to start restoring power until the area was declared safe, energy provider Endeavour Energy said in a tweet. "Affected customers should consider alternative accommodation until their power is back."
Sporting body Cricket NSW urged clubs to consider cancelling weekend games due to poor air quality.
"We appreciate this may mean some finals are not played and players may be disappointed, however the welfare of our cricket communities is our number one priority," Cricket NSW CEO Lee Germo said in a statement.
Australia's worst bushfires on record destroyed thousands of homes in Victoria state in February 2009, killing 173 people and injuring 414 more.