To get an instructors rating in Oz, you will need to have an Oz CPL. No IR, Twin rating or ATPL's needed. I think you may be able to get an Oz CPL with around 180 hours or so but don't quote me on that.
The Oz CPL is straight forward and not as tricky as the UK CAA/JAA.
------------------
"Jerry, just remember, it's NOT a LIE if YOU believe it." - George Costanza