Qantas says 26 passengers who began vomiting during a flight from Chile to Sydney contracted a gastro bug before boarding the plane.
More than 300 passengers were on board a 14-hour Qantas flight when it departed Santiago sometime after midnight (AEST) on Thursday.
About three hours into QF28, 26 people from the same tour group began vomiting.
They were moved to the back of the plane where four toilets are located.
The plane landed in Sydney at 5.38pm (AEST) on Thursday, cleared the runway but was parked short of the international terminal.
The affected passengers were treated and the remaining passengers were transported by vehicles to the terminal.
Six NSW Ambulance crews were on standby to assist.
Quarantine crews boarded the plane to inspect the interior before it was to be moved to the terminal.
"It is believed the illness was contracted before boarding the aircraft and symptoms became evident during the flight," Qantas said on its website.
"As a precaution, other passengers have been advised to keep a close eye on their health over the next 24 to 48 hours and seek medical advice if they become unwell."