Air New Zealand passengers had a terrifying wake-up call this morning when an emergency was declared on board their aircraft over the Pacific Ocean.

Morgan Kelly was on an overnight flight NZ5 from Los Angeles to Auckland when in the middle of the night the cabin lights blinked on.

Oxygen masks fell from overhead, and a loudspeaker began blaring, "This is an emergency. This is an emergency. Put on your mask", she told the Herald.

Kelly, 23, is a frequent flyer between LAX and Auckland and said the flight had been "totally uneventful" as usual, with minimal turbulence.

The wake-up call was "insane" Kelly said, and passengers were left shaken.

"All the flight attendants have to snap into it as if it was an emergency. Everybody was so shocked, including them - it was quite crazy."

She saw at least two people having panic attacks including one who collapsed in the aisle, leading to requests for a doctor.

Everyone put on their masks, she said. "Then it was a harrowing 15 minutes of nobody knowing what was going on."

Kelly said she was struggling to recall some of the incident, as she had blurred it out. But she remembered playing out in her mind the possibilities of what could happen.

"I think in my mind I was expecting to freak out. It was just 'Bury it, bury it'. I was listening to the sound of my heartbeat," she said.

"We listened to the captain calling to the flight deck, asking us to put on our masks and follow instructions.

"He finally announced that they had double-checked everything and it seemed fine."

However, the masks were left hanging for the rest of the flight - Kelly understood they needed to be manually put away.

Oxygen masks dropped and an emergency was declared on Air NZ flight from LAX to Auckland early this morning - but it was a false alarm. Photo/ Supplied

"The captain mentioned that lots of the flight attendants had never dealt with this before - oxygen masks only deploy if there's significant altitude drop or really severe turbulence."

The flight was still three hours from Auckland and there was no nearby airport to divert to. While the captain and crew were reassuring, they did tell passengers oxygen would keep flowing from the masks as a precaution.

"People were quiet - but quietly scared," she said. "It was three hours to land, and with them telling us about the weather in Auckland people were gripping their seats a little tighter."

The captain was extremely apologetic to passengers after touching down just after 5am, Kelly said.

"I've never been more thankful to be on the ground."

Morgan Kelly, 23, has taken the flight from LAX many times but this is the first time there's been even a minor incident. Photo / Supplied

An Air NZ spokeswoman confirmed there had been an incident on this morning's flight, adding that the captain and inflight services manager had kept passengers informed with an announcement coming about two minutes after masks were deployed.

"We are also in the process of getting in touch with customers on the flight to apologise for the disruption."

A statement from the airline's chief operational integrity and safety officer Captain David Morgan said oxygen masks were automatically deployed as the aircraft descended from 34,000 feet to 27,000 feet to avoid forecast turbulence.

"During this descent an automated emergency warning activated requesting customers put on their oxygen masks," Morgan said.

"We are sorry for the alarming wakeup call on this flight. This was not an emergency situation and the oxygen masks were not required. While our cabin crew and pilots worked quickly to reassure everyone on board, we know it was distressing for our customers."

Engineers were inspecting the cabin monitoring system of the aircraft this morning to find the cause of the fault, and it would be fixed before the next flight.