TYRE SIZE TROUBLE

Hi all.
Where do i start. I have changed my stock tyres to AT's. I opted for the larger size 265/75-R16 from guidance and advice from my 4x4 mechanic and tyre manufacturer/rep. Standard size was 255/70-R16. This was done because a light truck tyre was not available in the 255/70-R16.
This has increased the diameter by 40mm. Big deal - I thought...

Now, I was not aware of all the implications this brings. Having read many 4x4 mags, every 3rd photo you see has monster wheels. I think it de-sensitised me and probably many people.
There are Vic road laws stating maximum tyre size change of + or - 15mm.
I read a post from somebody yesterday saying they have an engineers certificate certifying the tyre change. Where do you get this?
I spoke to my insurer and they requested this but they did not know where to get it done. My mechanic has never heard of this being asked by anybody.
Another expert said, just don't worry about it as they would have to prove the tyres were the cause of the accident.( I think I would have to prove they did not cause the accident at my expense)
Anybody had this done in Victoria before? Anybody heard of this being done before in Vic?
And finaly - if so where? I am in Melbourne.

Thanks