Plane passengers got a sneak peek at confidential redundancy letters as a company administrator sat reading them, leaving them behind when she went to the toilet.

The company, AMP, is restructuring after a merger in March, and a spokeswoman said consultations about affected employees had not been completed.

Passenger Frank van der Zwaag said he was on the Auckland to Wellington flight last Wednesday evening when a woman sitting near him pulled out a stack of papers marked "private and confidential".

She was wearing a badge marked "AMP", and her personally-addressed letters also carried the company name, Mr van der Zwaag said.

"I could clearly see it was about an HR department restructuring.

"AMP staff can contact me if they want to know which letter they will get before they are posted."

The woman left the letters in her seat pocket when she went to the toilet, Mr van der Zwaag said.

"I thought, this is amazing. I'm sitting there, looking across the aisle, and she pulls out the letters from a plastic sleeve and sits there quietly reading.

"I thought, 'Gosh, you just don't do that'. But then I thought, 'Oh well. This is great entertainment. This is a great test case'."

He said someone who worked at the company or knew one of the affected workers could have seen the letters.

"Having gone through redundancy myself, I know that's not how you want to find out."

An AMP spokeswoman said the company was investigating.

"This is the first AMP has heard of this situation," she said. "We take the protection of private and confidential information very seriously and we appreciate this matter being drawn to our attention."

The company formed by the merger of AMP and AXA had been putting together a new organisational structure, and this process would affect some positions, the spokeswoman said.

Employment law expert Kathryn Beck, a partner at Swarbrick Beck MacKinnon, said the incident gave rise to many potential legal problems.

Redundancies were common after a merger, but companies had to follow procedures and there could be issues if they had drawn up termination letters too early, she said.

"If my name was on a termination letter and a friend had looked over the shoulder and saw it, first of all it's a huge breach of privacy," Ms Beck said.

"But if I thought I was only partway through a process and had a shot at the job going, and yet there's someone else sitting on a plane with my termination letter, it shows the process was a sham ... That would be a really bad look for them, as well as mortifying for the person involved."

People often let their guards down about confidential matters in airplanes and airport lounges.

It was inappropriate to read private documents where other people might be able to see them.