The Transport Accident Investigation Commission has opened an inquiry after a flight bound for Sydney from Wellington yesterday experienced an "engine issue".

Flight NZ249 was diverted to Auckland yesterday afternoon following the incident. The plane landed safely.

While Transport Accident Investigation Commission (TAIC) chief investigator of accidents Naveen Kozhuppakalam said it couldn't confirm the cause of the incident, initial reports suggested an engine malfunction did occur.

"We're calling for anyone who was on board and noticed something unusual to contact the commission in confidence," he said.

"Getting the facts straight is vital, so we're keen to hear as soon as possible from people who were on the aircraft.

"TAIC was informed that the crew turned the aircraft towards Auckland, issued a mayday, then landed safely at Auckland. We understand everyone on board disembarked safely and we have received no reports of injuries."

Kozhuppakalam said TAIC's initial focus is on evidence which could disappear or change, including memories which are fresh in peoples minds.

"We're also interested in the individual design, history, performance, maintenance, and equipment of this particular plane and its engines."

People who have information which may help the investigation can email TAIC at [email protected].

TAIC said it opens an inquiry when it believes the circumstances of an accident or incident have — or are likely to have — significant implications for transport safety, or when the inquiry may allow the Commission to make findings or recommendations to improve transport safety.

Its purpose is to improve transport safety by avoiding repeat accidents, rather than by ascribing blame.

Today, passenger Ruth Ann Herd told RNZ's Morning Report the inflight entertainment stopped while she was eating dinner.

"And then we dropped, then we descended and it was quite a rapid descent.

"I looked out the window and I could see we were closer to the ocean then before and then we did a big swing and turned in another direction and I thought oh, oh, we're being diverted or something."

Passengers told Herd they heard a grinding sound from one of the engines.

"Someone thought they saw a bit of something coming away from the plane or smoke or smoke trail of some kind," she said.