Thanks people! Sandwich plate will be being used Watajk, that's for sure.
that being said, if the cop is in a dickish mood, and cant find anything else, hell probably find it.
I hear ya. The forward vision rule is fairly easy to adhere to, and I cant see how a gauge is any different in that sense to a hood scoop, aerial, or any other item that is added to a vehicle - but I am sure that if I get the right officer he will be happy to explain why I am wrong and give me a sticker.

Does anyone know what the actual place to find the regulation so I can have a look at it? I have trawled through vicroads and national road website but cant find anything related directly to this.

I know this has happened to many people with gauges mounted above the dash, but apart from the excuse of saying it will cause an issue during an impact (which adding mirrors to your car will also but is legal) i cant find any legislation directly to the illegailty of of it.