Passengers and crew from separate Qantas and Air China flights have been quarantined in Shanghai over swine flu fears.

A Qantas spokeswoman said Chinese authorities diagnosed a passenger on flight QF129 from Sydney with the virus when it arrived in Shanghai on the evening of June 17.

As a requirement of local authorities, 39 passengers and four crew members were quarantined, the spokeswoman said.

"Qantas is monitoring the situation closely," she said.

She said the passengers and crew were expected to be quarantined in a Shanghai hotel for seven days as a requirement of Chinese health authorities.

Meanwhile, about 40 passengers on Air China flight CA178 from Melbourne to Shanghai were also confined in Shanghai hotels after a passenger on the flight was found to have an elevated temperature, Fairfax Media reported.

Luke Hamilton told Fairfax his sister-in-law Louise Atkinson, of St Kilda, was one of the Air China passengers quarantined after one passenger sitting near them was found to have a high temperature.

He said attendants checked the temperatures of passengers an hour before the plane was due to land.

"The whole process and procedure seemed pretty weird; I don't know how they decide who to quarantine," he said.

"She's upset obviously, she said she was quarantined but the people in front weren't."