I built my last cellar underneath a large deck. It was essentially a cool room structure with a Fondis cooling unit in one of the walls (to keep temps at 14 degrees instead of 4). It worked well, with the cooling unit's compressor not needing to kick in all that often. (With the deck overhead, it was in permanent shade.)
You could in theory do the same thing within a shed - however, the "outside" temp for the Fondis cooling units is a maximum of 35 degrees otherwise you risk damage to the compressor. (The 35 degrees would be the temperature inside of the shed. Unless the shed is shaded,I imagine most places around Australia would regularly have temps exceeding 35 degrees in the summer months.
kookaburra75's suggestion above would work. If you weren't confident doing it yourself, you can get prefabricated concrete structures which are cellars. Waterproofing would be needed, and you could build a shed over the top of it to conceal the entrance. Depending on where you live, there still might be temperature fluctuations inside the cellar over the seasons.