Thanks for the speedy reply, as you can see from my message to Grant, I have now ordered the book.
As for the bike buy-back, I have been dealing with a company called Car Connection Australia who are based near Melbourne. Their website is www.carconnection.com.au
I discovered them from the Lonely Planet Australia book but their bike tours company also gets a mention on this site. The address for the tours company is www.biketours.com.au
Although it's a buy-back scheme in name it's effectively a long-term rental agreement but with you being solely responsible for the bike during the period of usage. They have been in business for 20 years now, so they must be doing something right, and the dealings I've had with them so far have been very professional. Alternatively if you are flying to Melbourne anyway you can organise a true buy-back deal with one of the shops on Elizabeth Street. If you walk down this street and trip-over you'll probably fall into a motorcycle shop! Also I can highly recommend the hostel Toad Hall on the same street so everything is nicely in walking distance.
I haven't arranged anything for New Zealand yet as I'm still not sure how long I will spend in Oz. However I can point you in the direction of the company New Zealand Motorcycle Rentals, website www.nzbike.com
, who seem to be a good outfit (also mentioned in Lonely Planet New Zealand and on this website I think). See the article "Pure Fun" in June's Trail Bike & Enduro Magazine - TBM, NZ Motorcycle Rentals supplied the bikes. This article alone will get you in the mood for the NZ leg of your trip.
If you have anymore questions feel free to contact me directly by email via the link on this page.