(a) Costs discretions are truly discretionary: see Oshlack v Richmond River Council [1998] HCA 11; (1998) 193 CLR 72 at 84 and there are no absolute rules; the discretion must be exercised "judicially".
(b) The purpose of a costs order is to compensate, or indemnify, the person in whose favour it is made, not to punish the person against whom it is made: Ohn v Walton (1995) 36 NSWLR 77 at 79 per Gleeson CJ.
(c) Rule 42.19 of the UCPR does not give rise to a presumption that costs will be ordered against the plaintiff: Fordyce v Fordham [2006] NSWCA 274; (2006) 67 NSWLR 497; Foukkare v Angreb Pty Limited [2006] NSWCA 335 at [65]; Pentroth Pty Ltd v Kirschild Pty Ltd [2006] SASC 356; (2006) 96 SASR 129; Australiawide Airlines Ltd v Aspirion Pty Ltd [2006] NSWCA 365 at [53]; Bitannia Pty Ltd v Parkline Constructions Pty Ltd [2009] NSWCA 32. However, the rule does create a starting point by requiring "... the plaintiff must pay such of the defendant's costs ..." unless that outcome is displaced by a discretionary decision ("unless the court orders otherwise").
(d) The rule makes it clear that a court may order otherwise; but the burden is on the party who seeks to persuade the court that some other consequence should follow. Generally, there must be some proper justification, sound positive ground, or a good reason, for departing from the ordinary position: Fordyce v Fordham at [2] per Santow JA; Australiawide at [54] per Bryson JA; circumstances in which it has been held appropriate to depart from the ordinary position include where the proceedings have been rendered unnecessary by circumstances beyond the plaintiff's control; where the plaintiff achieved practical success in the proceedings, or where costs have been significantly increased by the unreasonable conduct of the defendant. If there is to be a departure it should be done in a particularized, and principled way.
(e) The plaintiff should be the moving party on an application for an alternative costs order: Bitannia at [70] per Basten JA. If facts are to be relied upon to found the court making a different order, the plaintiff will bear the onus of proving the relevant facts.
(f) Where the proceedings are discontinued prior to any hearing on the merits, "the Court cannot try a hypothetical action between the parties" to determine the question of costs: Australian Securities Commission v Aust-Home Investments Ltd [1993] FCA 585; (1993) 44 FCR 194 at 201; Minister for Immigration and Ethnic Affairs; ex parte Lai Qin [1997] HCA 6; (1997) 186 CLR 622 at 624; Metro Chatswood Pty Ltd v CRI Chatswood Pty Ltd [2007] NSWSC 1120 at [35]. At the time of discontinuance, usually it will be impracticable to assess the eventual prospects of success in the action.
(g) It may be necessary to analyze the whole of the proceedings to determine the appropriate costs order: Fordyce at [67] per McColl JA. A relevant consideration is whether the plaintiff acted reasonably in commencing the proceedings and whether the defendant acted reasonably in defending them: Australian Securities Commission v Aust-Home Investments Ltd at 201 (cited with approval in Foukkare); all the relevant circumstances, and not just the fact of discontinuance, should be considered; thus, the reasons for the discontinuance can bear heavily on the exercise of the discretion as to costs: McClure v City of Stirling (No 3) [2009] WASC 247 [4]; O'Neill v Mann [2000] FCA 1680; Beeson v Carrello As Liquidator of Gecko Management Pty Ltd (In Liq) [2010] WASCA 155 at [13].
(h) In a particular case, it might be appropriate for the Court, in its discretion, to consider the conduct of the defendant prior to the commencement of the proceedings where such conduct may have precipitated the litigation: Foukkare at [66] per Beazley JA;
(i) It is also important to draw a distinction between cases in which one party, after litigating for some time, effectively surrenders to the other, and cases where some supervening event, or settlement, so removes, or modifies, the subject of the dispute that, although it could not be said that one side has simply won, no issue remains between the parties except that of costs. In the former type of case, there will commonly be lacking any basis for an exercise of the court's discretion otherwise than by an award of costs by the successful party. It is the latter type of case that usually creates problems, since there may be difficulty in discerning a clear reason why one party, rather than the other, should bear the costs: One.Tel Ltd v Deputy Commissioner of Taxation [2000] FCA 270; (2000) 101 FCR 548 at 553; cited with approval in Edwards Madigan Torzillo Briggs Pty Ltd v Stack [2003] NSWCA 302 per Davies AJA (with whom Mason P and Meagher JA agreed) at [5].
(j) The distinction between the two categories referred to above is often helpful in exercising the costs discretion, notwithstanding that neither category can be precisely defined, the boundary between them is unclear and other factors may be relevant: Bitannia per Basten JA at [79]-[81]; Perre v State of New South Wales [2009] NSWLEC 51 at [49].
(k) Where the proceedings are discontinued after interlocutory relief has been granted, the court may take into account the fact that that interlocutory relief has been granted: Australian Securities Commission v Aust-Home Investments Ltd at 201.
(l) There is a risk that the subjective motivations of the plaintiff in discontinuing may be put forward as a basis for some other order. Except to the extent that such views may have been put before the defendant, for example as a basis for settlement, and are established as such on the evidence, subjective considerations of one party will generally be immaterial, so that the discretion will be exercised on the basis of the objective circumstances established on the evidence: Bitannia at [81].
(m) The court is required to make such order as it thinks just in the particular circumstances of the case.