Postcodes in Australia were rolled out in 1967, before the idea of a city at Monarto had been proposed. Hence just one postcode for the entire area. There is no postal facility for Monarto North, East, West or Central. At the listed coordinates you are standing in front of a Community Postal Agency. It measures about 2m x 2m. The post office at the Tea Tree Plaza devotes more space than this to Andre Rieu DVDs alone. Facing the main road you will see a number of post boxes, with another fine example of creative number sequencing by Australia Post. The information you seek is based on the standard/envelope size boxes, not the parcel ones.
ABC is the number of the box between #1 and #3 (surprise!)
DEF is the number of the box above and slightly to the right of #16
The cache is located a walkable distance away at:
S 35º 07.CBB E 139º 07.(B + F)(E - F)0 Checksum 19
Note that the cache had to be moved slightly because of obliteration of the original hide host Thank you Skuzzybear for helping out
Take care with the busy roads in this area. Replace the cache with care remembering that the stamp needs to stay with the cache.