At the end of each day, the balance in these accounts is paid an interest rate of 25 basis points (a quarter of one per cent) below the cash rate. If a bank finds that its projected balance is below zero, it borrows from the Reserve Bank at an interest rate of 25 basis points above the cash rate.
So banks have a strong incentive to manage their liquidity so that they have just enough funds, but not too much, in these accounts.