Heya Gina...
From a quick Google I've found these two that have SM/TBA available in Sydney...
http://www.bikescape.com.au/http://www.rollingthunder.com.au/ (advertises as HD rentals but also has a SM)
No experience with any of those companys so can't help there. However if you're coming as far north as Brisbane I'd love to show you round.
Remember that March/April is the beginning of winter here and some parts get really cold (have a look at the pics of my ride last week!) so bring/hire cold & wet weather gear. We also legally have to wear helmets in every part of the country. Get yourself an international driver's licence before you come and make sure it's endorsed for bikes. Finally make sure you ride the Putty Road north out of Sydney - it's THE bike road in the country.
