Sixteen of 26 Qantas passengers struck down with vomiting and diarrhoea on a flight from Chile to Sydney have been taken to hospital.
Three had to be stretchered from the plane.
Qantas believes those affected contracted a gastro bug before boarding.
More than 300 passengers were on the 14-hour Qantas flight when it left Santiago sometime after midnight (AEST) on Thursday.
About three hours into QF28, members of the tour group began vomiting.
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It is understood they were students returning from the international Catholic event, World Youth Day, in Rio de Janeiro.
They were moved next to the toilets at the back of the plane and attended to by nurses and at least one doctor who were passengers.
The aircraft landed in Sydney at 5.38pm (AEST) on Thursday and cleared the runway but was parked short of the international terminal.
Seven paramedic crews were dispatched following a triple-zero call about 20 minutes before the plane landed, NSW Ambulance said in a statement.
"Paramedics assessed 26 patients on board," it said.
"Of those, 16 patients required transport to hospital, three of whom were stretcher patients."
Quarantine crews boarded the plane to inspect the interior before it was 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."
