Major banks are secretly prying into customers’ spending – including the amount of petrol going into the family car – to identify those who are heading toward a home loan default well before they miss a repayment.

One technique, using technology developed by London-listed Experian, involves identifying borrowers who used to fill up their petrol tank to the top every time they visited the bowser – but now are only paying a round number and driving away with less than a half-filled tank.