Home Building Contracts Act 1991

A builder must not enter into a contract that contains an amount or an estimated amount for a prime cost item or a provisional sum if the amount or estimated amount is misstated by being less than the least amount that it could reasonably cost to supply the item or perform the work to which the amount relates.

In determining whether an amount is misstated for the purposes of subsection (1) regard is to be had to the matters or contingencies that were known (which may be set out on the contract), or ought reasonably to have been known, to the builder at the date of the contract.