I have spoken to Revenue SA twice this morning regarding this just to satisfy myself as I have quite a few clients that this could impact on. Funnily enough I have been given 2 different answers, one saying that it is date of contract for new builds and one saying that it is the completion date.
Interestingly the second person that said date of completion for new build also stated that it is date of settlement for established!
Therefore I have emailed Revenue SA and asked for the correct information to be confirmed in writing.
One thing that they all agree on is the amount that you would be entitled too if you were able to obtain permanent residence within the allowable backdated timescale which is the amount of the FHOG and bonus grants etc that was available on the date you signed either the contract of sale if established or the building contract if a new build.
As soon as I get the information clarified in writing I will post it on here.