A Perth-bound Airbus A330-200 carrying more than 220 passengers returned to Sydney Airport shortly after take-off on December 15, 2019, because of a hydraulic leak.
As the aircraft arrived back at the terminal under tow, a haze began to form in the cabin and flight deck, and passengers and crew started to get irritated eyes and throats.
Nearly 130 of the passengers evacuated via aerobridges, while more than 90 used escape slides.
A number of passengers used the slides in a way that risked injury, the Australian Transport Safety Bureau said.
Ultimately, one passenger was seriously injured and ruptured tendons in both their knees. Another five sustained minor injuries including knee sprains, frictions burns, and cuts and abrasions on their elbows.
Qantas cabin crew didn't practice phrases like "leave everything behind" and "jump and slide", an investigation by the bureau found.
There were also inconsistencies with the airline's safety video and briefing card when it came to how people should use the slides.
The pre-flight video showed a passenger placing their bag next to them before going down, and the bureau said some on the Perth flight brought their cabin baggage to the top of the slides.
Passengers collecting their baggage likely slowed the evacuation process, it said.
"While some complied with cabin crew and left them behind, others were shown on CCTV with their luggage in-hand, after using a slide," bureau Chief Commissioner Angus Mitchell said.
"Passengers should always leave their belongings behind during an evacuation."
Qantas has since amended its briefing video and was looking to incorporate "leave everything behind" into its primary evacuation commands, the bureau said.
The commissioner also pointed out that some cabin crew members detected unusual smells both before and after the aircraft was towed back to the terminal, but failed to alert the flight crew.
"Communication between the cabin crew and flight crew is essential in abnormal situations, and it is important for information to be relayed as soon as it becomes available," Mr Mitchell said.
The safety bureau noted Qantas did not have a procedure for rapid disembarkation at the time - something the airline advised it was reviewing in May of this year.
"In this case, given the information available and the physical symptoms being experienced by crew and passengers, the captain's decision to evacuate was a sound one," Mr Mitchell said.
Qantas has also since introduced periodic training that requires cabin crew to physically demonstrate evacuation procedures. Some of the 2019 crew did not rearm their doors before opening them during the Sydney Airport evacuation.
The bureau found the hydraulic leak happened because a rudder hydraulic hose ruptured during the flight.
After landing, the crew turned on the auxiliary power unit to maintain power, and the fluid was distributed out through the aircraft's air conditioning system.