Clause Sixthly
39 Mr Muddle did not develop the second proposition by reference to the authorities. However, he seemed to accept, for the purposes of the argument, that the Third, Fourth and Fifth Variations varied the terms of the Mortgage, as between the Company as mortgagor and the appellant as mortgagee, and that the Variations did not constitute separate contracts.
40 Whether or not I have understood Mr Muddle correctly, I think it clear that the Variations were intended, as between the Company and the appellant, to vary the Mortgage. As Handley JA pointed out in Credit Lyonnais Australia Ltd v Darling (1991) 5 ACSR 703, at 718, parties to a contract can always alter their bargain by varying or adding to its terms. A later agreement between parties to a contract may affect the contractual relations established by the earlier contract, but the later agreement may be interdependent with the earlier. (Handley JA dissented in the result in Credit Lyonnais, but not on this point).
41 In the present case each of the documents executed was described as a "Variation of Mortgage". Each was in registrable form and, if registered, would have enabled a memorandum to be endorsed on the Mortgage (cf Conveyancing Act 1919 (NSW), s 91). The only changes made to the Mortgage were to the interest rate, the principal sum and the term of the Mortgage (although none of the later Variations dealt with all three matters). The remaining terms of the Mortgage were unaltered. The provisions of Annexure A were unaltered, except insofar as the Variation affected the three matters to which I have referred. It is difficult to see how the Third, Fourth and Fifth Variations could operate otherwise than as variations to the Mortgage.
42 Mr Muddle also seemed to accept for the purposes of this argument that the Third, Fourth and Fifth Variations each altered the nature of the Guarantors' obligations and that the alteration could not be regarded as unsubstantial or non-prejudicial. Mr Muddle was correct to do so. The Third, Fourth and Fifth Variations each altered the amount borrowed by the Company. By reason of the increase, the Guarantors were exposed to a potentially greater risk of being called upon to meet a default by the Company of its obligations under the Mortgage. The Guarantors were exposed to a greater risk even if their liability was limited to the original sum lent to the Company ($240,000), plus interest. The increased borrowing by the Company may have made it more likely that the Company would default and that the Guarantors would be required to meet any shortfall (albeit up to a limit of $240,000). This is not a risk that can be dismissed as purely "theoretical" or the product of "passing fancies of judicial imagination": see Corumo Holdings Pty Ltd v C Itoh Ltd (1991) 24 NSWLR 370, at 380-381, per Kirby P; cf at 404, per Meagher JA.
43 It is open to the parties to a contract of guarantee to agree that a variation of the principal amount will not discharge the surety: Wood Hall Pty Ltd v Pipeline Authority [1979] HCA 21; 141 CLR 443, at 455, per Gibbs J (with whom Barwick CJ and Mason J agreed). Whether the contract of guarantee has that effect will depend on its terms, bearing in mind the principle that the liability of the surety is strictissimi juris and that ambiguous provisions should be construed in favour of the surety: Ankar, at 561; Andar Transport Pty Ltd v Brambles Ltd [2004] HCA 28; 217 CLR 424, at 433 [17], per Gleeson CJ, McHugh, Gummow, Hayne and Heydon JJ.
44 A principal debtor clause, if sufficiently broadly expressed, is capable of excluding the Ankar principle. For example, in The Fletcher Organisation Pty Ltd v Crocus Investments Pty Ltd [1988] 2 Qd R 517 ("Crocus Investments") a mortgage contained the following provision:
"6 … In order to give full effect to the provisions of this instrument the Guarantor agrees and declares that the Mortgagee shall be at liberty to act as though the Guarantor were the principal debtor and the Guarantor hereby waives all rights in connection with such provisions that it might otherwise be entitled to claim or enforce."