Frequent flyer programs were originally designed to reward passengers for sticking with an airline.

But then airlines noticed how much people valued the points and realised they could make money by selling them.

"About 10 years ago Qantas realised that there was an opportunity for them to commercialise this more widely and it was largely because of how consumers value those points both as a status symbol and as the opportunity to get flights," Credit Suisse director of equity research, Paul Butler, said.

Qantas is by far the most popular reward program in the country.

There are 12 million members of the program and they earned more than 120 billion points last year.

Today Qantas has more than 200 "partners", which include the four major banks, supermarkets and insurance companies.

These partners buy frequent flyer points from Qantas, which they can use to reward their own customers.

Rewards programs have become a huge generator of cash for the airlines.

Its frequent flyer program brings in more than $400 million a year in profit for Qantas, which is more than it makes flying people overseas.

It is also a very stable money earner for an industry that is known for its volatility in earnings and is highly reliant on oil prices.

Credit Suisse calculates the reward business is worth about $4 billion to Qantas.

Qantas has by far the most popular reward program in the country. ( Supplied: Qantas )

Real cash now for future rewards

What is particularly attractive about this business for the airlines is that they receive real cash from their partners, but the points are not usually redeemed straight away and they control what they are redeemed for.

"The dominant reason it's exceptionally good for the airlines' cash flow is that they're really getting a revenue stream for a very little cost stream," chief executive of Airline Intelligence and Research, Tony Webber said.

He should know — he was the Qantas chief economist for nearly four years.

Frequent flyer points are a type of currency, with the airline effectively an unregulated central bank.

Airlines determine the value of the points by deciding how many to issue, how many points are needed to purchase an upgrade or flight and what additional fees or charges the customer must pay.

This gives the airline a huge amount of power to control its cash flow.

Technically, the rewards programs "buy" seats and upgrades from the domestic and international divisions of the company.

But it is the airline that determines how many and which seats are available, which means the marginal costs are low.

"Without someone upgrading into business class from economy class that seat will go unsold, so having a passenger sitting in that seat rather than having an empty seat flown doesn't add that much to the incremental cost to the airline," Dr Webber said.

"That additional cost of upgrading the passenger is immaterial to the airline."

Especially because passengers are slugged extra fees and charges in addition to the points when buying seats.

These include taxes and fees that are third party costs, such as airport charges, but the airline also adds a "carrier charge", which it determines and "reflects market conditions and varying demand".

The reward business is worth about $4 billion to Qantas, according to Credit Suisse. ( Supplied: Qantas )

Airlines 'really want the points to expire'

Frequent flyer points might be highly prized and people might store a great deal of value in them, but in the case of both Qantas and Virgin Australia's programs, they can expire.

Steven Wren was, as he puts it, a loyal customer of the flying kangaroo for 18 years.

"Nothing gave me more greater pleasure at an international airport than that big tailed plane. So, the way I felt about Qantas is I was proud," Mr Wren said.

For his loyalty, he had accumulated more than 300,000 points — enough to get him an around-the-world ticket in business class — but he was saving them for a specific purpose.

"My in-laws are originally from England and we lost my father-in-law many years ago and it was our intention to return his earthly remains to England to where he was born," Mr Wren said.

But earlier this year he logged into the Qantas system to find all his points had disappeared.

He had been stung by a rule where points expire if there is no activity (no points have been earned or spent) on an account for 18 months.

Mr Wren is not alone.

There are social media groups dedicated to people in similar situations.

People who for a variety of reasons have seen their accounts go dormant and their points wiped away.

In Mr Wren's case, Qantas stopped flying to the Gold Coast, replacing the full-service carrier with its budget alternative Jetstar, so he started using Virgin Australia instead.

For other people, it is because of a change of career, a new baby or an illness that can see them curtail their flying for a period.

Points with both Qantas's and Virgin Australia's programs can expire. ( ABC News: Andrew O'Connor )

Suzanne Anderson was a member of the Qantas Frequent Flyer program for more than two decades and used the airline to fly regularly around the country and the world for work.

Her family circumstances changed, so she stopped travelling as much but she was still shocked when she logged in to her account to see her points balance had been wiped.

It was not just her points that had disappeared — her two daughters, then aged 10 and 12, also lost their total 32,000 points.

Dr Webber said it was in the interest of the airlines for the points to expire.

"They really want the points to expire, they have a strong incentive to expire the points. As soon as the points expire there is no cost associated to the airlines with these points being earned," Dr Webber said.

Qantas 'challenges' customers to retrieve lost points

But, the airlines also do not want to do anything that damages the brand or the appeal of the program.

Qantas and Velocity can only sell points to banks and retailers if the points are appealing to customers, meaning airlines are unlikely to make drastic changes that could devalue the programs in the eyes of users.

"It's a huge amount of value that investors have a lot of confidence in and, if they were to do something in the short term that increases the earnings on that business by reducing the value of the points for consumers, then it's going to have a very significant impact on the value that investors believe the program has," Mr Butler said.

After multiple calls to Qantas, Ms Anderson was offered a "personalised" Qantas Challenge whereby she could get the points back if she earned 2,500 points with two Qantas partners over six months.

The ABC has spoken to other people who were offered their own challenge but with different point targets and timeframes.

Ms Anderson earned the points back but still thinks that Qantas could have handled the situation better by clearly warning her when the points were due to expire (she says it was buried deep in emails) or by being more understanding when she tried to get the points back.

"While we know it's frustrating for people when their points expire, it's really easy to stay active in the Qantas Frequent Flyer program. It's as simple as earning or redeeming one Qantas Point within 18 months," the airline said in a statement.