Writing a "recommendation letter" for a supervisor from a student?
I don't know about the Australian system; in the US, it's quite normal for applications for promotion and teaching awards to require letters from students (though I think usually students from classes, rather than doctoral students).  However, I wouldn't pre-emptively write one; my recommendation is that you write a letter or email, in which you could offer if they ever need such a letter.  The other thing you could reasonably do is write a short email to their head of department, just saying in a few sentences wy you felt that they were successful as a supervisor.  That's probably the best way to help their career.