Apparently relocation/living away from home rate of youth allowance depends upon >90 minutes by public transport, or your parents home is not an 'adequate place to study'. This is not an A1 source, apparently the Act only says you qualify for relocation if the Secretary determines that "the person needs to live away from home for the purpose of education, training, searching for employment or doing anything else in preparation for getting employment;", and they've used discretion to develop this particular standard, there's flexibility in their determination so. Waf probably said his parents beat him, that's what I did.
http://www.centrelink.gov.au/internet/internet.nsf/filestores/st088_1006/$file/st088_1006en.pdf
2 units per study period, 6 per year? same as 3 per semester?
I don't think it takes into account trimester system, if you're less than three for the main two semesters, you won't get the sticker regardless of the third. Full time student, and then withdrawing before paying uni fees, just means you're issued the sticker to get cheap public transport, but legally you're not supposed to use it, they would take it if they could, and centerlink will ask for their money back.