I expect you are already aware of this - but to bring your bike to Australia you will need a carnet. See the thread started by Singapore Dream back in August 2009 where he has a link to the official Australian Govt publication on that topic. As a carnet is only valid for 12 months you would need to check out with the issuing organisation how to get it renewed while you are away.
Hiring a bike for that period of time would be horrendously expensive, and I doubt they would let you take it to NZ, whereas buying over here would be relatively straightforward, though you would need to check out the licencing and insurance situation as you would have a IoM drivers licence.
I am not sure of the legalities of the State of Victoria which is where you will be if you are starting out from Melbourne, but I would think you could get a Victorian Licence over the counter on the basis of your UK one and, unlike the UK DVLA, Australian authorities normally allow you to keep your old licence. The Victorian RTA website should help you. Australian insurance companies will normally also credit you with your IoM no claims history if you bring documentary evidence with you.
Last edited by Deolali; 29 Aug 2010 at 14:00.