Every time you buy something with a debit or credit card, your bank learns a little bit more about you. It already knows your personal details age, address, phone number, employment status, marital status, income, etc.

And increasingly, thanks to the decline of cash and rise of card payments, it also knows all about your spending habits. It knows whether you shop in 7elveven, 99 speedmart, and how often you eat out, how much you spend on petrol, whether you drive or get public transport to work, how often you go on holiday, and so on.

What the bank does with that information, as long as it remains within the limits of legality, is up to the bank.

The big banks are tracking and recording in minute detail every card transaction you make, and at least two of the big four. Research shown Westpac and the Commonwealth Bank are making extra money selling that information to third parties.

One of the most famous reporter found that Westpac is selling anonymous, aggregated information about customers’ spending habits to other businesses through a data sharing platform called ‘Data Republic’, which it jointly owns with NAB, ANZ and Qantas.

Commonwealth Bank, meanwhile, is selling anonymous, aggregated information about transactions to its business banking clients. NAB informed The New Daily that while it does not sell such information at the moment, it is considering doing so in the future. ANZ said it did not sell such information. Qantas, however, confirmed it sells what it called “de-identified” data to third parties over the Data Republic platform.

There is nothing illegal about any of this, and both Westpac and CBA stressed they were not sharing any personal information about their customers, and that any information shared was done so securely.

Still, this discreet practice is likely to be of interest to customers of Australia’s two biggest banks, especially in the light of the ongoing scandal embroiling tech giant Facebook and data firm Cambridge Analytica.