The original cache went missing after six years. I suspect the nearby landowner was a little purturbed by the extra traffic on this quiet country road.
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Across the road from GZ you will see four structures in a paddock. Do not enter the paddock, the owner is likely to get a little upset with you.
These storage vessels are one of a number of sites from WW2 as RAAF "Inland Aircraft Fuel Depots".
There are 3 in-ground, steel lined, concrete fuel tanks of 500,000 litres capacity each (being 10m diameter by 25' or 7.62m high). The 4th smaller tank (positioned slightly off-set NW'ish) was apparently used for ethanol storage (which presumeably was used for octane boosting the fuel, as required by blending, much like E10 fuel sold nowadays).
The tanks are connected via tunnels 6'x4' which took pipes to/from tanks to the small, adjacent pumping station. Fuel was delivered to/from to tanks via nearby rail. This tankfarm as built in 1942 at a cost of £42,000. It was decommissioned in 1946. They were originally almost covered by dirt. Recently a lot of work has been done to tidy the site in preparation for a new venture; conversion of the tanks to 3 story houses.
There are a number of other RAAF "Inland Aircraft Fuel Depots", the closest being at Tocumwal. This is apparently still being used for fuel storage. It looks like this in aerial view...