Manish
The error in your spreadsheet is that the two gst amounts are being added when they should be subtracted and vice versa.
Ok here's a GST on development 101 - this is incredibly high level and absolutely needs to be discussed with your accountant before you do anything, sign anything, apply for anything, etc.
If you're building townhouses with the intention of selling them, you're carrying on a business and have to pay gst on the selling price. So if you sell the townhouses for $550k each, you have to hand over $50k each to the ATO. However, you will have reclaimed all of the gst you paid to the builder, architect, etc, but that will be a lot less than the gst on the selling price.
However, subject to some specific situaitons, the margin scheme may apply. If you buy a piece of land or a knockdown house for $500k, then build two townhouses and sell them for $600k each, so $1.2m in total, then the margin is $700k, ie the difference between what you paid for the site and what you sell it for. If the margin scheme applies, then the gst you pay to the ATO is 1/11th of $700k, rather than 1/11th of $1.2m, so saving you about $45k. You also get to reclaim all of the gst you pay the builder, etc.
If you develop townhouses with the intention of holding them as rental properties, you're not performing a GST activity and therefore can't reclaim the gst you pay the builder, etc. However, if you sell one or more of the townhouses within 5 years of building them, there's a chance that you may have to pay gst on the purchase price.
These two approaches - building to sell or building to rent are treated completely differently for tax.
If you build to sell, all revenues and costs are added together to see if you made a taxable profit or loss, which is taxed at your relevant marginal rates, or 30% if you're operating in a company.
If you build to rent and you're building it under your own name (rather than a trust) then the interest on all borrowings is deductible against your other income from the time you buy the land in the first place, as is land tax, rates, etc, but not any costs of building. Plus whenever you sell the townhouses, you only pay CGT on half the taxable gain, but this is worked out very differently from the build to sell method. However if you sell within 5 years of building them, you run the risk of the ATO saying your original intention was to sell, and that's a whole load of pain you don't want...although the rules in this space are changing at the moment.
Hopefully this hasn't confused you too much. It's a hugely complex area and you need to be very careful in how you approach your develoment and how you structure it, although if you've already bought the land then it's too late for that.
Cheers
Jonathon