I remember going to a Vic Vettes meeting one night where they had a guest speaker from the company that runs Victorian speed cameras. He said as soon as a new product comes out aimed to defeat the cameras they go out and buy it to test. He said those sprays don't work, the number plate covers DO work but also stand out.
The only product they tested, which is invisible, is a wax available in USA, and used by NASA (for some reason?). He wouldn't tell us the brand name or anything else about it.
He also said anytime VicRoads intend on issuing new types or number plates, they test them first to make sure the cameras pick them up OK. He said if he lived in the western district of Victoria, he would put Ballarat regional plates on his car as they snuck through the tests but were marginal and probably shouldn't have.