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.
That is true - but some land is just as expensive as a pre-existing house. For example, it is rare to be able to find a decent block of land up here on the coast for under $300,000 - whereas we could sell this house for around the same. And I can tell you that the stamp duty paid to buy this house would be a HEAP lower (especially as new thresholds have been introduced) than it would be to by a block of land.

Its on the value of the land for sure - but only works out cheaper IF you manage to get a reasonably cheap block of land.