New Zealand's national airline is asking passengers to step on the scales before they board international flights.

Air New Zealand says it wants to weigh 10,000 passengers during a monthlong survey so pilots can better know the weight and balance of their planes before takeoff.

But the numbers from the scales won't be flashing up for all to see. There will be no visible display anywhere, the airline promised, and the weigh-in data will remain anonymous even to airline staff.

“We weigh everything that goes on the aircraft — from the cargo to the meals onboard, to the luggage in the hold," said Alastair James, a load control improvement specialist for the airline, in a statement. "For customers, crew and cabin bags, we use average weights, which we get from doing this survey.”

Indeed the numbers are required by the nation's industry watchdog, the Civil Aviation Authority.

Under the authority's rules, airlines have various options to estimate passenger weight. One option is to periodically carry out surveys like Air New Zealand is doing to establish an average weight. Another option is to accept a standard weight set by the authority.

Currently, the authority's designated weight for people 13 and over is 86 kilograms (190 pounds), which includes carry-on luggage. The authority last changed the average passenger weight in 2004, increasing it from 77 kilograms (170 pounds).

Health statistics show New Zealanders are becoming heavier. The latest national health survey put the adult obesity rate at 34%, up from 31% a year earlier. Childhood obesity rates increased to 13%, up from 10% a year earlier.

Customers on Air New Zealand domestic flights were asked to weigh in a couple of years ago.

James said there was nothing for passengers to fear by stepping on the scales.

“It’s simple, it’s voluntary, and by weighing in, you’ll be helping us to fly you safely and efficiently, every time," he said.

The airline said the survey began this week and will run through July 2.