Passengers on board an AirAsia flight have described the terrifying moment when the plane lost cabin pressure and plunged 20,000 feet shortly after take off.

Flight QZ535 was just 25 minutes into its journey from Perth to Bali when it was forced to turn back because of a technical issue on Sunday morning.

Oxygen masks dropped from the ceiling as a precaution and the 145 passengers were told to adopt the "brace" position as the plane dropped altitude.

"I actually picked up my phone and sent a text message to my family, just hoping that they would get it," Leah, a passenger from Perth, told Nine News.

"We were all pretty much saying goodbye to each other. It was really upsetting."

Video posted on social media shows a member of the crew saying: "Passengers get down, get down!"