14 Attention was focussed in submissions almost exclusively on the rectification claim. This is not surprising, given the parol evidence rule and the very narrow scope it allows for the efficacy of an antecedent oral contract in a conveyancing transaction. The relevant principles were stated by Knox CJ in Hoyt's Pty Ltd v Spencer [1919] HCA 64; (1919) 27 CLR 133 at 139:
"(a) When parties negotiate an agreement by parol and subsequently reduce it to writing, the writing constitutes the contract ( Knight v Barber 16 M&W 66, at p. 69), or at any rate is conclusive evidence of its terms ( Wake v Harrop 1 H&C 202), subject, of course, to the right of either party to proceed for its rectification or rescission on sufficient grounds. (b) A distinct collateral agreement, whether oral or in writing, and whether prior to or contemporaneous with the main agreement, is valid and enforceable even though the main agreement be in writing, provided the two may consistently stand together so that the provisions of the main agreement remain in full force and effect notwithstanding the collateral agreement.
15 In the present case, it is the concluding proviso regarding consistency between the earlier agreement and the written form the parties in fact adopt that presents a problem for the Mannalls (see also Maybury v Atlantic Union Oil Co Ltd [1953] HCA 89; (1953) 89 CLR 507 and Gates v City Mutual Life Assurance Society Ltd [1986] HCA 3; (1986) 160 CLR 1). The parties obviously contemplated that the contract "negotiated" by the making of the highest bid and the fall of the auctioneer's hammer should be reduced to writing and signed. The creation of a written contract was not only required by item (h) of the auction conditions in clause 18(1) of the Property Stock and Business Agents Regulation 2003 but also necessary under s 54A of the Conveyancing Act 1919 to ensure enforceability. The whole of the context indicated a wholly written contract. Consistently with that, the parties acted immediately after the auction to sign a contract at $2,820,000 containing clause 13.2 in the form that had been circulated to persons interested in bidding.
16 The antecedent oral contract the Mannalls seek to set up is inconsistent with the written contract. It conflicts with the written contract's specification of the price as $2,820,000 and the statement in clause 13.2 about GST not being added to the price. The antecedent oral contract cannot be asserted consistently with the parol evidence rule.
17 Like counsel for the Mannalls, therefore, I concentrate on the rectification claim based on alleged common mistake. In doing so, I note that the legal rules are not in dispute. The basic principle was stated by Campbell JA in Franklins Pty Ltd v Metcash Trading Ltd [2009] NSWCA 407 at [444]:
"In considering whether to grant rectification of a written contract, equity does not use any of its own principles to decide what the terms of the contract are, or how they are construed - those matters are decided solely by the common law. Rather, equity focuses on what it is unconscientious for a party to assert about the contract. The rationale is that it is unconscientious for a party to a contract to seek to apply the contract inconsistently with what he or she knows to be the common intention of the parties at the time that the written contract was entered. In other words, when a plaintiff succeeds in a claim for rectification, the plaintiff is found to have been justified in in effect saying to the defendant 'you and I both knew, when we entered this contract, what our intention was concerning it, and you cannot in conscience now try to enforce the contract in accordance with its terms in a way that is inconsistent with our common intention'."
18 His Honour also said (at [446]):
"The remedy that is granted is, as with all equity's remedies, one that will seek to undo, so far as is in practice possible, the departure, that the litigation has shown to exist, from equity's standards of conscientious behaviour. The way this is achieved, when a remedy of rectification is granted, is by rewriting the contract so that it is no longer departs from the common intention of the parties. The rewriting is done in a quite literal sense - the proper form of order identifies the precise words of the contract that are to be struck out, the precise words that are to be inserted, and where those words are to be inserted: Seton's Judgments and Orders , 7 th ed (1912) vol 2, p 1638-43."
19 Campbell JA added (at [447]):
"That this is the type of remedy that is granted has an effect on the sort of 'common intention' that is relevant for rectification. The common intention of the parties has to relate to what the mutual rights and obligations of the parties will be, and has to be sufficiently well-defined and clear to be able to be stated in words that can be incorporated in a contract."
20 Authoritative guidance as to the correct approach to a rectification claim of this kind is provided by members of the High Court in Pukallus v Cameron [1982] HCA 63; (1982) 180 CLR 447. Wilson J said (at 452):