The financial regulator issued the warning around mortgage offset accounts, which are marketed to borrowers as a way to whittle down mortgages and cut interest payments. In one case, a customer paid $3500 in higher interest payments in one month because their offset account was not linked to their loan. The relatively small losses identified by the watchdog did not match the complaint volume about banks mismanaging customers' accounts, comparison website Canstar said. Offset accounts are also a particularly attractive target for scammers because of the vast sums of money Australians collectively keep in them, Ms Tindall said. "Keep all your banking details incredibly safe, particularly if you've got large amounts in your offset account," she said.