The shortfin mako (Isurus oxyrinchus) is a fast-swimming pelagic shark of global ecological and socioeconomic importance, yet its trophic ecology remains poorly understood in temperate eastern Australian waters. We investigated ontogenetic and spatial variation in the feeding ecology of shortfin makos using stable isotope analysis (SIA) of nitrogen (δ¹⁵N), carbon (δ¹³C), and sulphur (δ³⁴S) from white muscle tissue collected from individuals captured during recreational game fishing tournaments along the New South Wales coast between 2005 and 2025. Stable isotope signatures varied with body size, indicating ontogenetic shifts in resource use. δ¹⁵N values declined with increasing body size, while δ¹³C values increased, suggesting that larger sharks rely more heavily on coastal prey occupying lower trophic positions than smaller conspecifics. In contrast, δ³⁴S values remained consistent across size classes, indicating predominantly pelagic foraging throughout life. Isotopic niche models revealed partial overlap among size classes, with increasing niche convergence in larger individuals, suggesting greater resource sharing at maturity. Seasonal variation was weak, likely reflecting both the slow isotopic turnover of shark muscle and uneven temporal sampling. Overall, the population exhibited a broad isotopic niche consistent with generalist feeding, although individual dietary specialization may occur. These findings demonstrate that ontogenetic changes in habitat use and foraging behaviour should be incorporated into fisheries management. The apparent increased reliance of larger individuals on coastal prey suggests they may be more susceptible to recreational fisheries than previously recognised. This study provides the first sulphur-inclusive stable isotope assessment of I. oxyrinchus and the first species-specific trophic framework for temperate eastern Australia, highlighting the value of integrating stable isotope analysis with electronic tagging, prey isotope baselines, and stomach content analyses to better resolve trophic pathways, strengthen ecosystem-based fisheries management, reduce bycatch risk, and support conservation of this endangered species.