Subdivision and GST Help!

Hi guys,

I have some questions in regards to subdividing my land in Doncaster and putting 3 townhouses on it. Here is the scenario after speaking to all the parties (builders, architects etc...)

1) Big block of land was purchased for $840k (around $890 after all the misc costs) few years ago.
2) Wants to sell each townhouse for around $900k
3) Construction cost is 990k (inclusive of GST) for all the units combined.
4) The property is under our personal names and we have been advised to registered a company and GST to carry out the development.

Let's say at the end we really managed to sell the luxury townhouses for $2.7m in total. How is the GST we need to pay calculated? Is it something like this?

Land cost - 890k
Construction cost and Sales commission - 1.05m
Sales - $2.7m
Margin - $1.81m
GST on Margin - 165k
GST on Construction and Sales commission - 95k

Final GST payable = 165k - 95k = 70k

Gross income = $2.7m - $1.94 (890k + 1.05m) = 760k.
Net income = $760k - $70k = $710k

Are my calculations correct? Seems to be a bit odd to me. Can someone kindly advise? If it is not feasible, we will develop the land later. Thank guys.
Setting up a company to undertake the development doesn't make sense (with the details provided) - why can't you just get an ABN & register for gst?

Total revenue: $2.7m incl gst, gst collected = 1/11 x $2.7m = $245k
GST paid (construction) = 1.05m x 1/11 = $95k
GST Payable = $150k

Gross Profit = $2.7 - $0.15 - $0.890 - $1.05 = $610,000 or 22.5%

(Providing that you're registered for gst)
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I have a very similar scenario with a 3 bed townhouse development. Please let me know if you gain any light on these calculations!!!