This is the second (and only one remaining) in what was a series of two caches called the "Greater Melbourne Traffic Islands" (GMTI) series, placed on those "wasted" plots of land around which vehicles travel; some of which can be quite large, or interesting in other ways.
This cache is a 200ml Sistema container, hidden in a circular "park" within the on-ramp loop from Hoddle Street northbound onto Heidelberg Road. The "park" is an isolated oasis amongst the traffic, including a small pond.
To get to GZ, approach from the south-west and find a way under Heidelberg Road. There is no need to cross either Heidelberg Road or Hoddle Street.