The 5118 postcode covers most of the Gawler region. Today, lets focus on Willaston. The original post office opened in 1864. The listed coordinates take you to the present day multi-service post office for Willaston. There are numerous post boxes on the internal and external walls. Use these to find what is currently the only letterbox cache in Gawler LGA.
Let ABC be the number of the box below #191
Let DFGH be the number of the box above #4
The cache is located at:
S 34º 35.(B + G)(D + H)(F + B) E 138º 45.CAF
Checksum 24.
You could walk to GZ from the post office, or alternatively move your car a bit closer, but make sure you don't put a river between yourself and the cache unless there is a bridge. Keep an eye out for muggles and their dogs, and be snake aware in warmer months. Enjoy the hunt, replace the cache appropriately, and as always, the stamp stays with the cache when it's a letterbox cache.