You will most likely have to keep digging until you find solid ground, them's the rules, we have had to dig some holes about 1.5m deep because of varous problems on site, but most of Sydney you will strike clay at around 300-400mm deep almost guaranteed.
Not much use waiting until summer as if the soil is no good, then it's no good, irrespective of time of year, from the sound of it 46 holes must be a decent structure going on it, then you will need a good solid base to support it.