Hidden details about the interest charged on partly-paid or overdue credit card balances is unfairly costing customers, consumer advocate group Choice says.

A study of 20 credit card companies showed the amount of interest charged on a credit card can depend as much on when a provider stops and starts charging interest and how fairly they apply interest-free days as the actual advertised interest rate.

American Express, Bankwest, Commonwealth Bank, ANZ and Westpac were named as the most unfair credit card providers.

“Many consumers would be surprised to learn they could have two cards with exactly the same interest rate and use them in the same way yet have one charging twice as much interest than the other if they pay late,” Choice spokesman Christopher Zinn said.

“The tricks of the trade make it much harder to compare the relative merits of different credit cards because the headline interest rate is only part of the story.”

Most credit card companies backdate their interest to the date of the purchase if a repayment is late, Choice said, meaning just one day late can result in higher interest being charged retrospectively for up to 55 days.

Partial repayments are also unlikely to stop that backdating occurring, Choice said.

For example if a customer were to underpay a $2000 bill by just $10, the extra interest would still be charged on the whole $2000.

Fairer credit card providers, such as Bendigo Bank, Heritage Building Society, Teachers Credit Union and some GE cards, only charge interest on the shortfall, Choice said.

Mr Zinn called on all credit card providers to use the same charging methods, and for customers to be aware of the finer details.

“It’s a simple matter to tweak systems to employ fairer systems but while most customers don’t understand the tricks they will inevitably continue,” he said.