A US-bound flight from Sydney turned back on Tuesday after a note found on board raised fears of a bomb threat, Australia's government said.
No device was found on the plane
Flight UAL840, carrying 246 passengers and bound for Los Angeles, landed safely back in Sydney airport nearly three hours after take-off.
Police superintendent Peter O'Brien later branded the threat a hoax.
He said all passengers had been interviewed and released, and the plane would resume its journey on Wednesday.
"We have been carrying out our investigations since the plane
landed and I'm quite satisfied that it is a hoax," Mr O'Brien told reporters.
Transport Minister John Anderson earlier said that a note had been found that carried "some words that implied that there might have been a bomb on board".
He said the threat was scrawled on either a sick bag or envelope and was found near one of the aircraft's toilets.
Sydney Airport was put on high alert
A Sydney airport spokeswoman earlier denied a local television report that an attempt had been made to break into the cockpit.
The flight took off at about 1500 local time (0500 GMT) and flew for 90 minutes before the pilot decided to turn round, a spokesman for United Airlines said.
It landed at about 17.45 local time (0745 GMT) in a secure part of the airport and was surrounded by security forces.
Passengers would resume their flight to Los Angeles at 1130 local time (0130 GMT) on Wednesday, United said.
The scare follows mounting security concerns in Australia.
Over the weekend, a group calling itself the Islamic Tawhid group and the European branch of al-Qaeda threatened to attack Australia with "columns of rigged cars" unless it withdrew its troops from Iraq.
And there has been alarm at a number of recent security breaches at Australia's airports, including an alert at Brisbane airport in May when a passenger wandered into a sterile zone without passing through security.