According to the ABS the Australian population at June 2019 was 25,364,300 people of which those aged 65 or older was 15.9% or 4,032.923 people.
If it costs $100 a day to quarantine those 4 million people for three months until the COVID-19 threat subsided it would cost $9,200 a person or a total of $37 billion. A lot cheaper (a third) than the $120 billion the government have estimated and saves lots of businesses, even more jobs and preserves the tax base with which to pay it back!
https://www.abs.gov.au/AUSSTATS/abs@.nsf/Previousproducts/3101.0Feature%20Article1Jun%202016