OP, since you are concerned about tax liability for both purchaser and seller, I'm assuming you're purchasing from a family member or friend? And the seller was not a property developer?
Firstly, sellers location doesn't matter for CGT purposes, as the asset falls under the category of land located in the state. However it might be relevant if a gift is involved for example.
It might help to put some rough numbers on this.
What was cost of site / value at transfer? This will be part of sellers base cost for CGT.
Roughly how much did they spend on developing?
What would open market value of the unfinished house be now?
How much are you buying it for?
With CGT, the price you are paying is not always relevant. If the value is higher than that, the seller needs to use value in calculating, and the difference is a gift to you. It could work to your benefits if the value is less than cost (which is not uncommon with unfinished property).
If you do a search here or Google you will find other posts on CGT calculation for self builds. It's not uncommon not to have receipts, but unless there was a lot of cash involved, seller may be able to demonstrate large transfers / cheques from their bank account that can help establish cost.
Apart from tax, make sure yourself that the house was built to plans, that planning is still valid if works are not complete, and that you understand changes in building regulations that might impact you in completing works.