Hi @Dix,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Unfortunately, this would be a question you'd have to pose to a structural engineer.
With processed materials, they are given a structural grade such as MGP10 or MGP12 with pine or F14, F17, F22 or F27 for hardwoods. As your hardwood would be being milled on-site, it would not have a structural grade that you could use to compare it with a span table.
Unfortunately, we can only make recommendations based on materials with this grade, as they have been assessed by suitably qualified people to be at or above a certain standard.
You could certainly look for a span table for structural hardwood to give you a bit of a guide as to what size timbers would be required, but without that grading, you would be, at best, taking an educated guess about what you could use.
A structural engineer would also be the correct professional to advise you on what scale of footing you would need for your build.
Apologies that I couldn't be more help.
Jacob
