You only pay stamp duty on the land, the house isn't built yet so you can't pay stamp duty on it.

You should never pay the full amount before construction, ever.

I will find my contract, we built in Melb in 2008, but you pay progress payments at different stages of construction - frame, plaster, lockup etc.

We got a construction loans and drew on it at each stage as the payments were due, you I ly pay interest on the amount you've drawn from the load amount do it gradually increases until completion when you will be paying the full mortgage.

Look up 'home one' it's a good forum for building and renovating