Hi @ramanpal,
A warm welcome to the Bunnings Workshop community and thank you for your question.
The first thing you should do is confirm whether your local council has any requirements relating to the construction of a deck. If they do, then you will need to confirm the suitability of the design with a draftsperson as plans will need to be submitted to council for approval. It is likely due to the size, you won't need building approval, but it is always worth confirming with a quick phone call.
Typically, you would use a H4-treated post, in either pine or hardwood, that is either 100x100mm or 90x90mm.
You might like to check out the following articles for some further guidance and information.
Let me know if you have any further questions.
Jacob
