Section 58 of the Act provides:
58 Applicant not owner of trade mark
The registration of a trade mark may be opposed on the ground that the applicant is not the owner of the trade mark.
Note: For applicant see section 6.
As with all grounds of opposition, the onus rests with the opponent. If the evidence is unclear, the opposition fails: Colorado Group Ltd v Strandbags Group Pty Ltd (2007) 164 FCR 506 at  per Kenny J and  per Allsop J.
The owner of a trade mark is the person who has first authored and used the trade mark (or one at least substantially identical to it) in relation to goods and/or services which are the same kind of thing as the goods and/or services of the impugned trade mark or has first applied to register it in relation to those goods and/or services (whichever is the earlier): Tosca TravelGoods (Aust) Pty Ltd v Samsonite IP Holdings S.a.r.l  ATMO 39 from  (citations omitted).
In Shell Co of Australia Ltd v Rohm & Haas Co (1949) 78 CLR 601, Dixon J said (at 628) that “Authorship … involves the origination or first adoption of the word or design as and for a trade mark.”
Nothing less than substantial identity will suffice: Carnival Cruise Lines v Sitmar Cruises Ltd (1994) 120 ALR 495 at 513 per Gummow J.
As to whether marks are substantially identical, in Shell Co of Australia Ltd v Esso Standard Oil (Australia) Ltd (1963) 109 CLR 407, Windeyer J said (at ):
In considering whether marks are substantially identical they should, I think, be compared side by side, their similarities and differences noted and the importance of these assessed having regard to the essential features of the registered mark and the total impression of resemblance or dissimilarity that emerges from the comparison. “The identification of an essential feature depends”, it has been said, “partly on the Court’s own judgment and partly on the burden of the evidence that is placed before it”: de Cordova v. Vick Chemical Co. (1951) 68 RPC 103, at p 106. Whether there is substantial identity is a question of fact: see Fraser Henleins Pty Ltd v. Cody  HCA 49; (1945) 70 CLR 100, per Latham C.J. (1945) 70 CLR, at pp 114, 115, and Ex parte O’Sullivan; Re Craig  NSWStRp 28; (1944) 44 SR (NSW) 291, per Jordan C.J. (1944) 44 SR (NSW), at p 298, where the meaning of the expression was considered.
As to whether the earlier user’s goods and services are “the same kind of thing” as those for which registration is sought, in Colorado Group Ltd v Strandbags Group Pty Ltd (2007) 164 FCR 506, Kenny J said, at :
The owner of a mark does not have rights at large in relation to the mark. The effect of the 1995 Act, which, in this regard, is much the same as earlier trade mark legislation, is that a trade mark must be registered in respect of particular goods or services as set out in Schedule 1 to the Trade Marks Regulations 1995 (Cth): see reg 3.1, 4.4 and Sch 1; also the 1995 Act, s 19 and 27. Ownership by first use is therefore ownership (or proprietorship) in relation to the goods or classes of goods on which the mark has first been used. The owner’s right to registration in this circumstance is not limited to the identical goods or classes of goods but extends to goods or classes of goods “of the same kind”: see Jackson & Co v Napper (1886) 35 Ch D 162 at 178 and In re Hick’s Trade Mark (1897) 22 VLR 636 at 640.
The earlier use must, of course, be use “as a trade mark“: Moorgate Tobacco Ltd v Philip Morris Ltd (No 2) (1984) 156 CLR 414 at . As to what constitutes use “as a trade mark”, see here.
As to the amount of prior use required, in Winton Shire Council v Lomas (2002) 119 FCR 416, Spender J said (at ) that “a very small amount of use will suffice.” See also Thunderbird Products Corporation v Thunderbird Marine Products Pty Ltd (1974) 131 CLR 592. In Buying Systems (Australia) Pty Ltd v Studio SRL (1995) 30 IPR 517, Gummow J accepted that preparatory steps coupled with an existing intention to offer or supply goods in trade was sufficient to support proprietorship of a mark. Preliminary discussions and negotiations about whether the mark would be so used, however, is not sufficient: Lomas v Winton Shire Council  FCAFC 413 at .