Especially if you're buying privately, you need to be sure the seller actually owns the motorbike you've set your heart on.
In NSW and QLD you can access REVS for a fee. There is a new database called the Personal Property Securities Register that covers everything, including bikes. It's Australia-wide.
It's cheaper than REVs. Up to $9.90 depending on the gateway you use. You can read more at:
http://www.ppsr.gov.auST Owners are all honest, but it's the low life that know a good bike when they see it, and steal it to get money for their drugs. Now you've been warned.