I doubt if you would need a Diploma yet,as I have not heard of anyone in the airlines having a diploma in aircraft maintenance,although I am sure the leeches in the TAFE/Aeroskills arena would love to squeeze more money out of anyone associated in aircraft maintenance in Australia.As it stands at present, Aviation Australia are charging AMEs & LAMEs $14K for upgrading your qualifications to unrestricted B1, so God only knows what they will slug a potential B2.
At present you only need a trade certificate,which a Cert IV is,so basically keep a log of anything you do,signed by a certifier. Combine that with an aircraft type course(e.g. B737,helicopter,etc) with your SOE for that type,and you are on your way,which I imagine most LAMEs will tell you takes a few years,even with OJT or PCT or whatever you want to call it.
Best of luck