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im pretty sure even the tram inspectors have the same right to do this, and good ol nanny state victoria would surely have this ability to demand for ID.
There is no 'law' that states that a human beings needs to provide jointure to their 'person' for a 'police man', thus the 'police officer'.
You as a human (sovereign being) are not obliged to answer any questions from a 'police officer'. You are obliged to ask which law and authority they are operating under.