Stirling,
The part of the property that is already built is the structure or building part which you rightly say depreciates over 40 years from the time construction is completed.
The question here is whether the construction was completed. I would suggest you have bought a project in the middle and therefore you haven't yet got a completion date.
The commencement date of construction is used to determine what legislative period the works falls into, but the construction completion, or date the property is available for rental is the start date for depreciation.
Hope this helps
Michael