Qantas has confirmed a flight that had to be diverted en route from Brisbane to Townsville was having cabin pressure problems.
The flight, QF974, was expected to land in Townsville in north Queensland at 3.50pm (AEST) on Tuesday.
It was instead diverted to Rockhampton in central Queensland.
Qantas wouldn't say initially why the diversion occurred, but later on Tuesday released a statement saying the crew noticed the Boeing 737-800 "was not maintaining its normal cabin pressure".
"In line with standard procedure, the captain initiated a descent to 10,000 feet (3km)," the statement read.
"Passengers were advised that because the aircraft wasn't maintaining its normal cabin pressure, they would fly at a lower altitude and divert to Rockhampton instead of continuing to Townsville."
Flight tracking data shows the aircraft dropped from about 40,000 ft (12.2km) to 10,000ft (3km) in about five minutes.
Qantas said passengers were "calm" throughout the descent and there were no reports of anyone on board suffering any adverse effects.
Engineers examined the plane at Rockhampton and a replacement aircraft was en route to take passengers to Townsville.
The incident has been reported authorities and Qantas said it would work with Boeing to investigate the case.
An Australian Transport Safety Bureau (ATSB) spokesman told AAP it would be investigating.
"Obviously, we'll be looking at it very closely and in due course," he said.