tax for new townhouses:how is tax calculated

Depends on your own individual tax position and also the property that you purchase. If subsequently the margin scheme can apply to your development.

If so generically

If you have a end sales value of 2 million. Your max gst could be as high as 200k
Applying the margin scheme. You pay roughly 10 percent of the difference between purchase price, development and build costs. And then the sale price. Ie bought 500k land, built 1 million. Make 500k profit pay 10 percent of that 50k split over the 4 houses 12.5k gst per house.

Then after you pay gst you pay based off your own individual tax rate. Which is variable.

And if property held in a company then your flat 29-30 percent.

Sometimes better to split sales over 2 financial years if your only development. Or a period where one partner might not be working as much. To take advantage of that years Lower income.
And or retaining the equity in one property and keep as rental.

The above is pretty basic. Speak to a accountant. With semi accurate development costs land costs and sales costs. To work your liabilities