I think you are getting mixed up with ICAO recommendations
Dick,
It is not a recommendation, it is a Standard. See ICAO Annex 6 Vol I Paragraph 6.18.2. It requires that turbine aircraft over 5700kgs
or authorised to carry more than 19 pax shall have ACAS (TCAS) II. The number of passengers is covered and it additionally covers large aircraft that do not carry passengers.
You are correct that Australia is lagging behind that ICAO standard. I do not know why.
None the less TCAS, shall not be a determining factor in deciding on Air Traffic Service (ATS) levels.