Thats like saying the police can arrest someone whom they believe is going to commit a crime, except its not even the police. Would you like to be arrested later today because someone believes you are going to drive recklessly on Saturday? and then they can hold you tomorrow, because you still might speed on Saturday. . . .From what I saw all of the worrying stuff is only in place during a declared state of emergency. Essentially, what I gathered of the detention bits are:
1) "Authorised Officers" include a range of public service employees based on their role, as well as anyone that the Secretary (I assume of the Department of Health and Human Services?) appoints. The Secretary can make such an appointment of anyone that they believe is appropriately experienced, competent, qualified, etc to undertake the role. ... This is the basis of the "citizens detaining citizens" concern - The Secretary is able to nominate anyone that they believe meets the criteria, not just people employed by the Victorian or Federal public service.
2) An "Authorised Officer" is able to detain, or order the detention of, any person or group of persons whom they have reason to believe are "high risk" and are likely to not comply with public health orders (ie isolation, quarantine, social distancing, masks, etc)
3) A "high risk" person is defined as someone who has been diagnosed with COVID and not yet medically cleared of the virus, and the notified (ie - they have to have been formally told by Victorian DHHS) close contacts of those people.
4) There are no conditions or limits placed on the above detention, with the exception of it requiring review every 24 hours by an "authorised officer" to confirm that it is still justified and appropriate.
So... It's not quite as "carte blanche" as some would like you to believe. It's not just any old person who can order someone be detained, they can't just order anybody at all detained, and they cant order them detained indefinitely without review... But, it is definitely a pretty strong extension of the existing powers.