It's only a matter of time before airlines start demanding fat passengers pay for two seats, an aviation commentator says.

Peter Clark said the number of very large people boarding aircraft was increasing and people who struggle to get into one seat should pay the extra cost.

His comments follow reports that Air France-KLM planned to make overweight passengers pay for a second seat.

The airline denies it would make obese passengers pay more and says the only recent change is they will refund the cost of the second seat, if one is bought, if the economy cabin is not full.

The extra seat came with a 25 per cent discount.

Mr Clark said airlines would soon have to start asking bigger passengers to pay for another seat or take a bigger one in premium economy or business class.

"The airlines will have to take a strong stand on it ... Something will have to happen. It's the responsibility of the passenger, not the airline."

He said airlines would probably try to form a consensus on the issue.

Safety issues were involved with very large people in planes, such as moving quickly in an emergency or even putting a seat belt on.

"A large person can create problems if they impede the movement of other passengers or if they have to evacuate quickly.

"It's a confined space and if someone is impinging on the space of another passenger - who has paid a lot of money for it - then that's not fair."

He thought there would be human rights issues if airlines did ask fat passengers to pay more but safety should come first.

Air New Zealand expects passengers who know they require extra room to buy another seat to get the space they required.

"At the time of the flight, if there are spare seats, Air New Zealand always tries to accommodate passengers requiring extra room," a spokeswoman said, "whether this is people travelling with children or for medical or personal reasons."