Welcome to another letterbox cache in the postcode series. When I was young this would have been in Munno Para LGA, today this will count as a find in the City of Playford.
Postal services in One Tree Hill go back to 1858, but the current office is situated in a modern building with two very well organised banks of post boxes. There are some older buildings along with a traditional cache just across the road.
From the listed coordinates you should be able to see two banks of post boxes, on facing north, one facing east. Use these numbers to work out the location of the cache.
Let AB be the number of the box to the right of #30
Let CDF be the number of the box to the left of #283
Let GHJ be the number of the box above #187
The cache is located at S 34º 43.(H - F)DJ E138º 46.BG(A - J)
Checksum of the minute decimals is 17
The cache is located a short way out of town to the east. It's a busy road so park carefully and supervise the kids. Nice views across Sampson Flat towards Mt.Gawler. Enjoy the hunt, and remember that the stamp stays with the cache for letterbox caches.