This paper investigates and critical analyses the market definition issues in News Ltd v Australian rugby Football League Ltd (19960 ATPR 41-466 and in Regent Pty Ltd v Subaru (Aust) Pty Ltd (1998) ATPR 41-647. Here the author examines the application of the provisions of the Trade Practices Act 1974 (Commonwealth) particularly sections 4, 45, 46 and 50 in relation to ‘market definition in Australia’. By attempting to critically analyse the issues of market definition within the per se rule and the rule of reason in an Australian legal sense, the author seeks an explanation to outline the key elements one needs to establish under the ‘substantial lessening of competition test’, ‘the notion of substitution’, ‘otherwise competitive with,’ and ‘the time factor’ in relation to market definition.