The big four banks have quietly taken the knife to savings account interest rates during the summer months, with many customers now earning less than 1 per cent on their cash held in the bank.

Commonwealth Bank, National Australia Bank and Westpac have all sliced some of the rates paid on their online savings accounts during January, following a cut from ANZ Bank in December, as competition between banks for household deposits eases.

Excluding introductory offers, interest rates on the big four's online savings accounts are now less than 1 per cent.

But as banks face fierce public scrutiny, cuts by three of the four banks were designed in a way that is likely to save the banks money, but attract less public attention.

That is because CBA, NAB and ANZ all reduced "base" rates, which are paid to long-term customers, while raising "introductory" rates, which are used in marketing materials but only paid to customers for the first three to five months after they open an account.