Jetstar engineers are investigating after one of the airline's planes was forced to make an "emergency" landing, with the cabin filling up with smoke.

The Airbus A320 was diverted to Brisbane during a flight from Sydney to Cairns after a "technical issue" prompted the pilot to shut down an engine.

The budget airline said at no point had an emergency been declared, adding that the smoke only came into the cabin via the air conditioning unit after the plane had safely landed.

However, a number of passengers dispute that.

Wendy Perkins said she heard a loud explosion, and smoke came into the cabin about an hour into what was supposed to be a three-hour flight.

"There was a large explosion under my seat and I didn't know what it was," she said.

"Within seconds smoke came up under my seat under my legs and up into my face and the man who was next to me.

"We both said that stinks, that absolutely stinks and then we lost pressure in the plane."

Some on the packed flight thought they were going to die.

A Jetstar flight from Sydney to Cairns was forced to land in Brisbane after an issue with an engine left the cabin filled with smoke. ( ABC News )

"The most terrifying moment of my life," passenger Aundra Thompson wrote on Facebook.

"I was just on the flight that caught fire and filled the cabin with smoke.

"Never have I cried so hard ... thankful to be alive and for the crew."

Passenger Nick Trompf supported Ms Perkins' version of events, however he said smoke appeared 45 minutes after take-off.

Loading

"The food cart sort of came up to the seat adjacent to me and smoke started to appear into the cabin," he said.

"The crew became fairly concerned and they immediately took the food trolley back to where it belongs and locked it away.

"The smoke was dense enough that it had a smell to it.

"It wasn't like that air conditioning mist that you sometimes get in a plane.

"It created quite a haze in the cabin and a smell with people wrapping jumpers and so on around their faces."

Cabin smelt like burning plastic

Another passenger, who was sitting over the wing, said "it was definitely burning inside the plane".

"It smelt like plastic or oil burning," Heidi Green said.

"The frightening thing was the horrible noises that persisted the rest of the 40 minutes of the flight.

Sorry, this video has expired Passenger films a Jetstar plane making an 'emergency' landing

"Once they shut off the engine the smoke began to clear.

"The landing was tense and the remaining engine was clearly straining but it was not turbulent."

After landing in Brisbane, the passengers continued their journey to Cairns after a four-hour stopover.

"Engineers are looking into the cause," a Jetstar spokesperson said.

"While it's rare for something like this to happen, our crew are trained to deal with these type of situations and they handled the situation very professionally."