Bird strike caused a Qantas flight from Queenstown to be diverted to Christchurch on Saturday afternoon, the Civil Aviation Authority says.

The Boeing 737-800 aircraft had just become airborne when the incident occurred.

Witnesses reported hearing banging noises. Video footage on social media appeared to show flames coming from an engine.

Nic Chamberlain was among those in Queenstown who saw flames and heard a series of loud noises.

"It was like just shoots of orange flames shooting out the back.

"You can tell it was coming out of the left engine," he told RNZ.

There was a delay before the loud noises followed, he said.

"This was kind of like rapid succession boom, boom, boom, boom, boom, boom. And then it was loud enough that everyone kind of turned their heads and looked to see what the noise was," he said.

Flight QF122 touched down safely at Christchurch Airport at about 5pm on Saturday. There were no reports of injuries.

RNZ understands about 160 passengers were on board.

A Qantas spokesperson said the incident was still under investigation.

"One of our flights to Sydney experienced a suspected engine issue after take-off from Queenstown this afternoon and the aircraft diverted to Christchurch.

"Our pilots are highly trained to manage situations like this and the aircraft landed safely.

"We know this would have been a distressing experience for passengers and will be contacting all customers to provide support," they said.

Hato Hone St John said it was notified of a flight diversion at 4.25pm and multiple ambulance resources responded to Christchurch Airport as a standard procedure.

A spokesperson said St John had not been required to assess or treat anyone, and all ambulance units had since been stood down.

Fire and Emergency New Zealand records show an aircraft emergency was reported at Invercargill Airport at 4.12pm, with Invercargill and Kingswell crews attending.

It is understood the flight was initially expected to divert to Invercargill before continuing to Christchurch.

Queenstown Airport confirmed the flight was diverted to Christchurch soon after departing and said the airport remained operational.

Source: RNZ