Police have thwarted a terrorist plot to bring down a plane, arresting four men in raids on homes in Sydney's suburbs.
Prime minister Malcolm Turnbull said security had been increased at Sydney Airport since Thursday because of the plot.
The measures were extended to all major international and domestic terminals around Australia overnight.
"I can report that there has been a major joint counter-terrorism operation to disrupt a terrorist plot to bring down an airplane," Mr Turnbull said.
"The operation is continuing."
Australian Federal Police commissioner Andrew Colivn said details were scant on the specifics of the attack, the location and timing.
"In recent days, law enforcement has been become aware of information that suggested some people in Sydney were planning to commit a terrorist attack using an improvised devise," he said.
"We are investigating information indicating the aviation industry was potentially a target of that attack."
Mr Turnbull advised travellers in Australia to arrive at airports earlier than usual - two hours before departure - to allow for extra security screening, and minimise carry-on baggage.
Justice minister Michael Keenan said the plot was the 13th significant threat disrupted by police since Australia's terror threat level was elevated in 2014.
Five plots have been executed.
"The primary threat to Australia still remains lone actors, but the events overnight remind us that there is still the ability for people to have sophisticated plots and sophisticated attacks still remain a real threat," Mr Keenan said.
"In light of this information, it's very important that everyone in Australia remains vigilant."
The operation was carried out by the Australian Federal Police, New South Wales state police and the Australian Security Intelligence Organisation, the country's main domestic spy agency.
The investigation could continue for days, Mr Colvin said.
Seven Network television said 40 riot squad officers wearing gas masks stormed an inner-Sydney house before an explosives team found a suspicious device.
A woman led from a raid by police with her head covered told Nine Network Television: "I love Australia."