This is what Consumer Affairs Victoria says :
Source here
This is what the Domestic Building Contracts Act 1995 Section 30 says :
contract, a builder cannot seek from the building
owner an amount of money not already provided
for in the contract if the additional amount could
reasonably have been ascertained had the builder
obtained all the foundations data required by this
section.
(8) Nothing in this section prevents a builder from
exercising any right given by this Act to the
builder to claim an amount of money not already
provided for in the contract if the need for the
additional amount could not reasonably have been
ascertained from the foundations data required by
this section.
Source here
What were your provisional sums for your site costs alone ????