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Just as a confirmation of what Rally and Brendan have been talking about here.
I poured my concrete footing in direct contact with limestone kunka. After drilling and digging down into it 1m x 1m x 0.8m. I ensured that the footing was 40 mpa (slump at pour) and specified from the concrete company. You can do 35 mpa with bunnings concrete but you need an extra bag or two of concrete powder and be mindful of how much water you add to the mix. 30mpa is readily achievable with the mix itself and water regulation. Too much water means a weaker concrete.
I then had a custom made pier which has a 8" OD with 12mm wall thickness at 1300 long. It has stabilising fins which come up 1/2 of the pipe length. Very stiff configuration. All bolted down with 16mm bolts.
I have tried hitting the pier when viewing at high mag and I did not get any movement whatsoever. It might deflect a bit but I have not seen anything in my images that resulted from pier deflection.
Bottom line is go as big as you can with all that. It saves digging up later and starting fresh. Like I said earlier go thickness and diameter. I was to do this all again I would probably try for a larger diameter pipe with the same wall thickness, just to really ensure stability.
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