Dramatic images and video has begun to emerge from inside a Qantas jet from which passengers had to evacuate via slides after the cabin began to fill with smoke.

Some passengers were injured as the incident took place in the skies west of Sydney on Sunday night. One man said he had "blood gushing out" from an injury sustained during the evacuation.

.@Qantas passengers have been caught up in a terrifying emergency evacuation at @SydneyAirport after what they described as smoke filling the cabin. The flight to Perth was forced to return to Sydney and three emergency slides were deployed. https://t.co/dL8YSRSnQQ #7NEWS pic.twitter.com/6EhAb8SGQE

— 7NEWS Sydney (@7NewsSydney) December 15, 2019

Qantas has said passengers experienced a "very unusual and probably alarming experience".

Flight QF575 took off from Sydney Airport at around 8.45am (AEDT) Sunday morning.

It was forced to return to the airport after the issue was detected, touching back down shortly before 9.30am.

Passengers evacuating from the Qantas flight. Photo/ Twitter, @EVILH0MER

The flight was forced to turn back after an issue with the plane's hydraulics shortly after takeoff.

The Airbus A330 landed safely but was unable to taxi and had to be towed to the gate.

"Once back at the gate, the captain made the decision to evacuate the aircraft as a precaution and three emergency slides were deployed," the airline said in a statement.

"Passengers were evacuated through both the slides and normal exits." Qantas insisted the incident did not fit its definition of an "emergency".

One passenger described the incident on Twitter as "terrifying".

"Everything was calm and then the captain said evacuate, evacuate, evacuate," one mum told Channel 9.

Aswin De Silva was sporting an injury after he took to the escape slide.

"I landed on my elbow and I saw blood gushing out."

Two passengers were treated for injuries sustained from using the slides and another passenger was taken to hospital as a precaution. Some customers reported sore eyes and itchy throats.

Video has emerged of passengers flinging themselves down escape slides and of what appeared to be smoke seeping into the cabin.

At least one passenger reported the cabin filling with "pungent" smoke before passengers were told to evacuate.

"Initially it was just the smell, we could smell something but after a few minutes this fume starting building up in the cabin," Sydney mechanic Rahman Akbari, 23, told The Daily Telegraph.

He said the captain appeared calm but it was a "little bit" scary when the crew began yelling "evacuate, evacuate" on the tarmac.

"Once the aircraft was back on the gate there were reports of a thick haze in the cabin, likely caused by hydraulic fluid entering the airconditioning unit. "While customers may have thought it was smoke, there was no fire," the airline said.

Qantas Fleet safety captain Debbie Slade said the airline understands the incident would have been a "very unusual and probably alarming experience for our customers".

We thank them for giving crew their full co-operation," Ms Slade said in a statement.

"The crew did a great job of putting their training into action and following the procedures for a set of circumstances like this.

"We'll investigate exactly what happened, including liaising with Airbus, before this aircraft is returned to service."

Engineers have inspected the aircraft.

But the emergency return to Sydney Airport may have in fact saved a life.

Flight industry veteran Clifton Brock told The Daily Telegraph he had been flying to Perth to care for his 97-year-old mum.

She called him after the unexpected landing to say she needed an ambulance, and the former aviation worker advised her to use a medical emergency button.

The call wouldn't have come through if her son had been in the air, as he would have been had the flight not returned to Sydney.

He said the plane "did a short landing and hit the brakes really hard".

"It was really fast, everyone had to be out in 90 seconds," he said.

At least some stranded Qantas passengers have now been placed on replacement flights to Perth.

Qantas cabin ‘filled with smoke’. Video / Kellguerard via Storyful

The Civil Aviation Safety Authority and the Australian Transport Safety Bureau have been notified of the incident, Qantas said.

The airline says it will assist the bureau with its investigation. "Qantas is providing support to customers on the affected flight and managing some delays to other flights that occurred as a result of this incident," the airline said.

"The airline extends its thanks to all customers for their patience and understanding."