A passenger onboard an Air New Zealand flight where an elderly man allegedly assaulted a flight attendant said he had to physically restrain the man.

Paul Donaghy "could hear a kerfuffle" while boarding flight NZ648 from Queenstown to Auckland last night.

"I think what had happened was that he'd been asked for his ticket and he wouldn't show it to the air hostess, he then tried to push past her... she was pretty diplomatic," Donaghy told the Herald.

"He tried to keep shoving past her, and then she held her hands up."

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"He brushed up another one of the air hostesses and that's when I jumped up."

Donaghy said the man appeared to be in a confused state.

"I quickly wrapped my arms around him to stop him from pushing anyone else and stop it escalating any further.

"A minute later the captain burst out of the cockpit and pointed his finger at the guy and said 'You're off'.

He said the man "roughed up" two female flight attendants, who were "distraught" in the aftermath.

"They were distraught once everything had happened, you could see how upset they were."

The captain of the Air New Zealand plane requested police and aviation security at 5.10pm.

A police spokesman said an agitated 75-year-old Kiwi man had pushed a flight attendant, causing her to stumble backwards.

"She was shocked but otherwise unharmed," police said.

The man was arrested for common assault but let off with a pre-charge warning.

"He said he was extremely remorseful for what happened," the police spokesman said.

A passenger on the flight, Steve Warne told the Herald he did not see the event that caused everyone to disembark the plane.

"What I find absolutely remarkable is the inconvenience and upset that this man caused to so many - and yet he is let off with a warning.

"There were those passengers panicking because they were missing connecting international flights, people with upset children and just people like myself who had been working in the town and wanted to get home to their families. This man's actions caused a raft of upset and he gets a mere warning?"

Warne said the man should have to foot a good share of the cost of the debacle.

"I think then, that he would think twice about pushing anyone else around in the future."

An Air NZ spokeswoman said police have confirmed there was an alleged assault on board the plane in Queenstown.

The incident happened as customers were boarding the flight.

Air NZ said customers were reaccomodated on alternative services.

Some passengers have already caught different flights, while others stayed overnight in Queenstown.

The union that represents flight attendants says assaults on staff are becoming increasingly common as more people fly to different destinations.

E tu Head of Aviation Kelvin Davis said there should be stiff penalties for people who misbehave on flights.

He said not only does it endanger members at work, it also endangers the travelling public.

Kelvin Davis said that's not acceptable.

Air New Zealand said it can't comment about an assault on one of its staff, because the police are involved.

- Additional reporting: Newstalk ZB