Ambiguity in the words of a written contract is not required before surrounding circumstances can be examined as an aid to construction. The law on subsequent conduct has been authoritatively settled by Agricultural and Rural Finance v Gardiner [2008] HCA 57 at [35]: subsequent conduct cannot be used to prove what parties meant by contractual terms, but events after execution may be admissible as retrospectant evidence of surrounding circumstances at the time of execution. For rectification, the common intention must be the actual or true common intention of the parties, established by clear and convincing proof, and the commercial context is relevant to determining that intention.
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