More than a million customers have stopped dealing with a financial institution in the last 12 months, with Australia and New Zealand Banking Group (ANZ) losing the highest proportion of the big four banks.
The Roy Morgan Research Consumer Single Source survey found 4% of ANZ's customers left the bank in the last year.
Westpac Banking Corporation (WBC) lost 3% of customers in the year, while Commonwealth Bank of Australia (CBA) lost 2.7% and National Australia Bank (NAB) lost 2.3%.
Of the smaller banks, BankWest lost 5.6% of customers, while ING Direct lost 5.2%.
St George lost 3.8% of customers in the year and Bendigo Bank lost 2.9%.
Industry communications director Norman Morris said banks could look at which financial institutions their lost customers are now choosing to deal and understand the reasons behind their choice.
"Our research shows the key reasons why people switched their main financial institution include high fees and charges, poor service and poor interest rates," Mr Morris said.
The study was conducted over six months to July and surveyed 25,132 people.