The Margin Scheme
This is the key. The greater the val on your land the less GST you pay. Have your land re-valued for Margin Scheme purposes when you are ready to commence – this means that it may well be valued far above what you paid for it. This represents a way to reduce the GST component of the final sale.

Also, dont forget that the GST payable on the sale is:
the difference between the land and the final gross realisation, less any GST component on your expenses.

As Ausprop said “you basically pay 1/11th of your profit in GST”. Just remember that this assumes you are getting GST credits on your project costs.
BTW, interest, bank charges and council charges do not attract GST. Neither does 2nd hand materials purchased privately.

Commercial Property:
A commercial property sold as a “going concern” is considered a taxable supply by a business. When you purchase such a property you recieve a GST credit on the entire purchase price! In this circunstance you do not elect to use the Margin Scheme. It is is up to the vendor to elect if they want to sell the property as a going concern or under the margin scheme.

5 Year Rule
Yes, GST is not payable on a developers property held for more than 5yrs. If it has been rented during this time it is treated as an investment by the developer and not as trading stock. This means that you can not cliam the benefit of GST credits while constructing the property. The sums need to be considered as each option has its own benefits.