Cheers for that guys,
John, Customs are obviously responsible for importing the bikes, but they need a Vehicle Import Approval (VIA) from the Department of Transport and Regional Services first, this document is incredibly dificult to obtain and quite impossible if you haven't had the bike for more than 12 months. There is no mention on any of the paperwork for the VIA of allowances for mileage unfortunately.
bmbiker, I hadn't thought of entering on a carnet and dealing with the rest later, but this does make sense in that I can get a certificate of compliance (after any necessary mods done on the bikes) and then deal with the Department of Transport. This sounds the logical route, and one I will seriously persue.
I had assumed that as I was ultimately importing the bike to Aus, then the last 'port of call' on the carnet would be the last country I leave to get to Aus. I'll check this with the chap from the RAC...
It's now all looking quite positive WOO HOO!!!!