Hawaiian Airlines' decision to weigh passengers on its American Samoa route is under investigation, after two passengers complained that it was discriminatory.

The airline has taken to weighing every passenger flying to or from Pago Pago, the capital of American Samoa, and is preventing them from picking their own seats online, Radio New Zealand reported. 

According to the airline, the decision to weigh passengers and assign them seats was taken to ensure weight was distributed according to Boeing 767 guidelines. Cabin crew are reportedly leaving one seat in each row empty, or occupied by a child passenger. 

An airline spokeswoman was also quoted as saying that data showed that flights to and from American Samoa were using a disproportionate amount of fuel. 

But two passengers have now filed complaints with the US Transportation Department, saying the airline policy is discriminatory as it only applies to flights to and from American Samoa. 

"Hawaiian is saying that it is a safety issue,” one of the complainants said, "so have we been flying unsafe for all these years?”

Hawaiian Airlines' decision to target its Pago Pago flights is most likely down to the fact that American Samoa has the world's highest rate of obesity.

Data for 2008 shows that 74.6 per cent of all adults there are obese, with more recent estimates claiming the rate is now above 90 per cent. The high rate has been attributed to the prevalence of a gene that increases the risk of obesity. 

In 2013, Samoa Air - the national carrier of American Samoa's neighbour, the State of Samoa - became the world's first airline to charge passengers according to their weight.