[9] WMC Resources concerned the construction of a contract under which the respondent performed open-cut mining work for the appellant. The contract provided that where its Schedule of Rates did not apply to a variation then, in the absence of agreement between the parties, the appellant "shall ... determine such value in its sole discretion". The respondent disputed the appellant's determination of the value of variations and the parties referred the dispute to arbitration. The arbitrator referred to the court the question whether he had power to substitute his own valuation for the valuation made by the appellant. Ipp J, with whom Kennedy and White JJ agreed, noted that although the contract empowered the company to determine the value "in its sole discretion", the critical feature of the valuation was that it involved a discretionary judgment.[8] A contract will often provide detailed fixed and objective criteria as to how the value of work is to be determined, providing for a mechanical exercise rather than a discretionary judgment.[9] In such cases, there will only be one uniquely correct value and a court or arbitrator can correct any mechanical error.[10] Sometimes, however, a valuer is required to exercise a very broad discretionary judgment.[11] Many subsidiary factors and contingencies may have to be taken into account so that there is a wide range of potential legitimate valuations of the work. In such a case, opinions may properly differ as to virtually all of the relevant issues to be considered in determining the valuation and as to the valuation itself.[12] There was an implied term in the contract that the appellant, in determining the valuation, would act honestly, bona fide and reasonably.[13] Ipp J concluded that in the contract for his consideration, the appellant's valuation was of the broad, discretionary type. The respondent could challenge it, only if it was not in accordance with the contract, including the implied obligation to act honestly, bona fide and reasonably (as explained by McHugh JA in Legal & General Life of Australia Ltd v A Hudson Pty Ltd [14]) or if the appellant's valuation was entirely erroneous (as explained by Mason J in Federal Commissioner of Taxation v St Helens Farm (ACT) Pty Ltd).