Seriously, a significant proportion of Australia's aggregate mortgage debt is in fact offset by deposits.
"n aggregate, the stock of Australian households’ prepayment buffers is estimated to be equivalent to over 10 per cent of the outstanding stock of housing loans (Graph B1). This includes balances in mortgage offset and redraw facilities. Flows into these accounts include regular excess repayments and one-off excess repayments paid out of salary bonuses or other irregular income."
