Fewer than 10 per cent of the 3.2 million people with a variable-rate mortgage at one of the nation's big banks have sought to boost their spending power since the Reserve Bank of Australia started cutting official interest rates.

In a sign the RBA's efforts to boost the economy with ultra-low rates may take longer to work, insights from the major lenders point to home buyers trying to run down their debt levels rather than going on a shopping spree.

Commonwealth Bank chief executive Matt Comyn revealed just 69,000 of its mortgage customers had reduced their repayments. Credit:Alex Ellinghausen

The big banks are facing the House of Representatives' economics committee to outline their responses to the banking royal commission. But both sides of politics have taken the opportunity to press the lenders on other issues, with committee chairman Tim Wilson asking how many customers had cut their repayments in response to the RBA's recent interest rate cuts.

The nation's biggest lender, the Commonwealth Bank, has about 1 million customers with principal and interest mortgages. It said just 69,000 of its clients had reduced their monthly repayments.