From:
https://www.casa.gov.au/standard-pag...pment-mandates
"From 6 February 2014,
any aircraft registered in Australia before 6 February 2014
that is modified by the installation of new or replacement ATC Transponder systems must have a Mode S Transponder capable of ADS-B 1090 MHz Extended Squitter transmission installed"
So unless the aircraft is
Modified by the installation (IE: New transponder model fitted under STC or Part 21 EO) then there is no need to install mode S.
So unless you are operating at major international airports or under IFR there is no need.