Have a look at postcode 4702. It extends from the Central Qld coast over 600km inland to past Longreach. The reason for this is, in the past, all small communities serviced by mail contract from, what is now Central Queensland Mail Centre, had this postcode. Over time, some of these communities have grown considerably, Gracemere and Emu Park spring to mind. Emu Park has now got its own postcode. Gracemere is still fighting for theirs. Some of these locations out west are nothing more than scattered properties, previously with a small community, which is now gone, but the area retains the postcode.
Generally, in Qld anyhow, postcodes were allocated in a fairly methodical order. Brisbane City area 4000 to 4004, Brisbane North suburbs, 4005 to 4099, Brisbane South suburbs, 4100 to 4199, Gold Coast area 42xx, Ipswich and to the west 43xx, Caboolture and the Sunshine Coast 45xx, Gympie up to Gladstone and out west 4600 to 4699, Rockhampton and to the west up to about Bowen 4700 to 4799, Bowen to the tip of Cape York 4800 to 4899. Obviously, as communities grow, and deserve their own postcodes, the number available reduces, so therefore anomolies occur. Remember, the postcode system commenced in the 60s, I think it was.
As my username suggests, I spent quite a lot of years in that employment, and it will take me quite a few years to match it with my railway career, if thats possible.