A firm with a market share of less than 35% can be found to be dominant with market power under section 7(c) of the Competition Act when crisis conditions (such as the Covid-19 pandemic) confer temporary market power that enables the firm to behave independently of competitors, customers and suppliers. Market power in such circumstances can be established by examining the firm's actual pricing conduct as a proxy. The concept of the 'lucky monopolist' applies where a firm's dominant position arises from exogenous events outside its control rather than from efficiencies or anti-competitive conduct. Once dominance is established and a prima facie case of excessive pricing is made out under section 8(2), the evidential burden shifts to the respondent firm to show under section 8(3) that the price charged was reasonable. To discharge this burden, the firm must provide credible justification that correlates with the extent and timing of price increases. Anticipated future cost increases, without clear evidence of their magnitude and timing corresponding to the price increases implemented, do not provide sufficient justification. The requirement that excessive prices be charged 'to the detriment of consumers or customers' in section 8(1)(a) is satisfied when essential goods necessary for health and safety during a pandemic are priced excessively. When determining administrative penalties under section 59, the de minimis nature of the contravention, the size of the firm, the limited harm caused, and costs already incurred are relevant considerations that may justify not imposing a penalty despite a finding of contravention.