A Qantas Airways flight landed safely at Sydney airport on Wednesday after issuing a mid-air mayday alert with engine trouble over the Pacific Ocean.

Ambulance services scrambled at Sydney airport to meet the incoming Qantas plane before it landed safely.

The flight – QF144 – was carrying 145 passengers from Auckland, New Zealand, a statement from Qantas said.

The Boeing 737-800 plane landed without incident on the runway at Sydney airport, said an AFP reporter at the scene, and will now be inspected by engineers.

Qantas said the mayday alert was downgraded to a PAN – or "possible assistance needed" – before the flight touched down.

The airline did not identify the problem precisely but said one of the aircraft's engines had "experienced an issue" about two hours after take off.

"While inflight engine shutdowns are rare, and would naturally be concerning for our passengers, our pilots are trained to manage them safely and aircraft are designed to fly for an extended period on one engine," it said.

Airservices Australia, the government's aviation regulator, said a PAN indicated a situation that "requires attention from air traffic control."

Qantas is consistently ranked among the world's safest airlines and has not been involved in a fatal accident for more than 70 years.

NSW Ambulance said earlier the service had initiated its "emergency activation" plans because the plane had made a mayday call while flying over the Tasman Sea between Australia and New Zealand.

Paramedics were called and ambulance services were placed on standby at the airport, a spokeswoman said.

The Boeing 737-800 is a twin-engine plane and able to land safely with just one engine.

According to the Australian government, a mayday call "indicates an aircraft is in grave and imminent danger and requires immediate assistance."