The Reserve Bank will be under pressure today to explain why interest rates on credit cards remain so high when the official cash rate is at an historic low.

Senior RBA officials including assistant governors Frank Campbell and Malcolm Edey will be grilled by a Senate estimates committee amid concerns that commercial banks are pocketing recent rate cuts.

While the RBA makes regular appearances before parliamentary committees, it is the first time the RBA has been summoned by a budget estimates committee.

The "spread" or margin between credit card rates and the RBA's official cash rate - now at a historic low of 2 per cent - is at its widest since 1990 when records began.

Australia's big four banks, which control half of the credit card market, face questions on whether they are "gouging" consumers with some credit cards charging around 20 per cent interest.

Labor Senator Sam Dastyari has warned that a wider inquiry into credit card rates could be initiated if the Reserve Bank is unable to justify the gap the cash rate and credit card rates.

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"What we've seen time and time again is that interest rate cuts, falls in the cash rate have not been passed on to credit card holders and frankly it's time we got some answers," Senator Dastyari told the ABC's AM program.

"What I'm worried about and what we need to get to the bottom of is whether we have a situation where there has been gouging going on constantly by the banks when it comes to credit cards."

Senator Dastyari said this afternoon's hearing will give the Reserve Bank an opportunity to explain the perception that banks are benefiting from RBA rate cuts which have fallen by 2.75 per cent points since late 2011.

"The big banks have made a lot of money at the expense of credit card holders. Let's not kid ourselves here," Senator Dastyari said.

"This is a very profitable part of Australian banking and, while there's nothing wrong with people out there making a buck, we have to make sure there is a proper competitive environment and that Australian consumers are not being ripped off."

The Australian Bankers' Association, which represents the big four banks, has rejected the "gouging" claims.

"Credit card interest rates vary considerably and will be determined by factors such as the risks of unsecured lending and the various features a card has," a spokesman told the ABC.

"The Reserve Bank of Australia cash rate has marginal impact on credit card interest rates."

The ABA pointed to research showing the average amount accruing interest per credit card in Australia has been falling consistently for the past three years.

It says Australians are "taking care not to pay as much interest" and that since late 2014, the amount accruing interest per card has fallen back to levels not seen since the beginning of 2007.

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