To settle once and for all the Victorian law on window tint, i went and discussed it with VICROADS
I was told in victoria its not very clear cut, however I was directed to the Vehicle Standards Information; Window Tinting 1 page document.
I was told by vicroads that roadworthys do check windows, but the windows that are ahead and to the side of the driver, this means that the rear windows are excluded.
I believe someone else mentioned vans and their ability to black out rear windows, vicroads also pointed out the same thing.
All in all, i was given the all clear to have darkened rear windows, but just for my own benefit take it to a windows tester and measure the visability.
Hope this is able to help anyone else.