Postcode Distances
Generally, suburbs that are near each other have postcodes that are in the same ballpark. Since postcodes are only 1 dimension, and locations are 2 dimensions, there are obviously going to be plenty of exceptions to the rule. But not so many that you can't make a graph like this:
So if you have two postcodes which are 10 apart (say 2000 and 2010), then on average they should be 105km apart (assuming you are working with Australian postcodes).
The graph can be drawn so it goes further in the x-direction, but things get a bit silly then:
Towards the very right, there aren't many datapoints at all (there aren't many postcodes 8000 apart) so it is actually quite accurate there, it just doens't look as cool.
I sourced my postcode data from sixfive.co.uk, processed it with python (since I'm using it at work, I figure I might as well do more with it) and plotted it with gnuplot. I also worked under the assumption that 1 degree of longitude or latitude is 111km, which I think is about right for most of Australia (though I was doing this from memory, not wikipedia). In any case, if you are using the actual numbers here for anything serious, then you are doing something wrong.
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