Greenhouse gas emissions from Western Australia's sheep flock account for 26% of the state's agricultural emissions, principally as a result of enteric methane emissions. A decrease in emissions between 2005 and 2019 can be partly explained by a 44% drop in sheep numbers over that period, but less is known about potential changes in the methane intensity of sheepmeat and wool kg CO2 equivalents/kg product. Using the livestock systems modelling software GrassGro™, we assessed the changes in methane intensity of sheepmeat and wool produced in two major sheep-producing regions in Western Australia. We also evaluated a series of future scenarios. Our results demonstrate that the observed emissions reductions are largely a result of a decrease in flock size, although methane intensity has also decreased somewhat by 11.1%. Simulation of future trajectories indicates that methane intensity could be as much as 18.8% lower by 2030, compared to 2005, with further reductions of up to 42% considered possible. The primary driver of the decreased methane intensity to date is increased flock reproductive performance through increased marking rates, higher rates of ewe lamb mating, and lower ewe death rates. However, despite reductions in methane intensity per kg of product, net emissions per ewe have risen 11.6% since 2005 and are forecast to rise by up to 21.8% by 2030, with potential further increases of up to 61% considered possible. This is driven by increased feed intake due to an increased number of lambs produced per ewe, higher ewe standard reference weights, and lower ewe death rates. Therefore, achieving absolute net reductions in the methane emissions through productivity improvements is unlikely to be prospective. Reducing net emissions is instead likely to be contingent on a reduction in flock numbers, breakthroughs in anti-methanogenic research, or via emissions offsetting. Our approach can be applied in other major livestock producing regions to evaluate emissions performance, with potential implications for agricultural and trade policy as markets increasingly seek lower emissions product.