'Gold bugging' is a hybrid between geocaching and gold/relic detecting.
Geocaching is a popular activity where GPS co-ordinates of hidden caches are published online and enthusiasts use their GPS to find them. Geocaching began as hidden stashes of small items where the finder takes one item from a cache and then substitutes it with another item that they leave behind. While those types of caches are still common it has expanded to hikers/bushwalkers seeking a secluded feature or a location, or maybe even just a message. It can be all of fun, challenging and tiring depending on the location and the difficulty in getting there and finding it. Many websites can be found on geocaching and the activity has gathered considerable interest worldwide.
There is no need to describe gold or relic detecting since, if you have found a gold bug, you would have used a detector to find it and know the fun of detecting already. And now you know what a gold bug is (that little stamped brass token you found) and what it's purpose is.
The major differences between geocaching and gold bugging are that the bugs are buried at various locations only within known goldfields, there are no GPS co-ordinates published, and that it requires the use of a metal detector. It will likely be gold or relic detectorists already doing their chosen activity who will find the bugs while detecting. This activity is not possible for those without a metal detector as digging random holes in the bush would be futile. But you don't need the latest and most expensive detector - any working detector even from 30+ years ago should find them.
All the gold bugs are located within the Victorian goldfields in Australia (with the exception of a few special edition bugs in other known gold bearing areas outside Victoria). More bugs are being buried upon regular trips to the goldfields.