I had that happen a couple of weeks ago with a suburb on the Gold Coast - I put in the postcode I had & I had to put Gold Coast to make it work.

I finished up printing my own label & paid at the PO (a little bit cheaper that way for 500 gram satchels).

Another problem with C&S, I created 2 items got 2 tracking numbers the same, I couldn't sort that one out, so paid over the PO counter instead.  

Can U imagine what would have happened if I had posted them with the same tracking number to different parts of the country?