It depends what you transfer from. If you transfer across departments (e.g. from science to engineering) or between universities, then (assuming things haven't changed since my time) you'd need to go through UAC just like you're doing now when you apply for university courses for next year, which would likely take into account your WAM as well as your ATAR. However, if you do an internal transfer within the engineering department (e.g. from civil to electrical or whatever), then I believe it should be handled internally by the engineering department and its up to them.
I had a friend who transferred from civil to aerospace, and when he approached the year coordinator with his transfer forms, he (the year coordinator) looked up his marks, and just went "hm... yeh alright" and signed his forms approving the transfer.
So, I would imagine transferring externally across departments is more difficult, since you need to go through the official channels and they take into account your ATAR, whereas internal transfers are typically more relaxed (and might not care about your ATAR).