Not up to date on all the states, but I know you save heaps in stamp duty in the ACT if you build, over buying pre-existing house. The reason being if you build, you only pay the stamp duty on the land component, not the actual building. whereas if you buy a pre-existing house you pay stamp duty on the lot.

Building can be stressful. But having bought a house and land package for our first house and now just signing contracts on another to be completed next year, I couldn't imagine owing a house for me to live in that isn't new. I wouldn't rule out a pre-existing house but I am naturally inclined towards 'new and shiny'.