The principal one is because we do all the Australian Government’s foreign exchange transactions. For example, when the Department of Foreign Affairs needs to pay its embassy in Washington in US dollars, the Bank sells the necessary US dollars to the government (in exchange for Australian dollars).
Generally, the Bank purchases the US dollars in the market prior to settlement, although it does have the option of funding its sale of US dollars out of its foreign currency reserves. On average, the Bank does around $ million of these transactions each day.