GF,
I agree that Canberra is an anomoly and here is why.
Since the min vis for flight under the VFR is 5000m then there really isn't any point in publishing a vis higher than 5000m for any aerodrome. If a vis of 8000m was published then in order to fly the approach the vis dictates that you would be in VMC anyway....So you have to be in VMC to fly an IFR approach....make sense?
Basically the maximum visibility that can be published for any aerodrome is 5000m, because this is the min vis for flight under the VFR.
So in the case of Canberra, if the vis was not below 5000m aren't you flying in VMC anyway? and so you can continue the approach, is that right?.....I am not familiar with ops at CB so excuse my ignorance if I am wrong.
Alpha