Hi @GrahamSev,
Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you with us.
If the deck is tall enough to require a balustrade, then it is likely something that you should be speaking to a builder or engineer for their recommendations. As balustrades need to be resistant to horizontal loads in line with NCC Volume One, Section D2.16 and AS/NZS 1170.1, the anchoring requirements of your posts would be fairly complex.
As a rule of thumb, posts should usually be set 1/3 of their total height within the ground. For example, if you want 2m of post out of the ground, there needs to be 1m in the ground, but you would need to confirm this is suitable with a qualified professional.
To be safe, I would encourage you to reach out to an engineer for their recommendations.
Let me know if you have any further questions.
Jacob
