A milestone is a numbered marker placed on a route such as a road, railway line, canal or boundary. They can indicate the distance to towns, cities, and other places or landmarks; or they can give their position on the route relative to some datum location.
In Australia they were typically White Stone Markers along our roads with two faces facing out, with a Black Letter representing the City in each direction and a Black Number advising the distance to each location in Miles.
When Australia went Metric these Milestones began to be phased out, and were replaced by the green trapezoidal sign with white letters/numbers that we see usually every 5 kilometres on our highways.
Despite being phased out many years ago, we can still see many examples of these Milestones around Australia. There are several in Goulburn.
There is one at the posted coordinates. There are three digits (a,b,c) on this particular one : Yab and Gc.
The cache can be found at the following coordinates :
S34 46 . (b*c) (a+c) (a-a)
E149 41 . (b) (b) (a+c)
BYO Pen.
Good Luck.