https://www.rba.gov.au/speeches/2020/sp-ag-2020-07-27.html  5/14  27/07/2020  The Reserve Bank's Operations – Liquidity, Market Function and Funding | Speeches | RBA  Graph 5  So the actions of the Reserve Bank have contributed to historically low funding costs for banks. Also, banks have benefited from an influx of low-cost deposits.
Some of that has come from businesses that tapped their lines of credit but then left those funds on deposit. There are a number of other reasons for the increase in bank deposits. This includes increased spending by the Australian Government (net of taxes), which leads to funds being paid out from the government's account at the Reserve Bank to customers of commercial banks.