I've done it but in NSW. I was 19 when I did it because my bio. father wouldn't sign the forms. It was just a matter of filling out a form with the births, deaths and marriages office and doing an interview. I also had to provide heaps of evidence to say i'd been using my mums remarried name for some time e.g school records, letters of support etc.
Feel free to PM me if you want more info.
WOW i must have had it easy as i also changed my surname at 19 but didn't need my parents consent (?), it was legal for me to do it on my own from 18, ... i just walked into one office on one side of the city and then another office on the other and 3hrs and $53 later my surname was changed ... but that was back in 1985, lol