Urban agriculture (UA) can be highly productive in terms of yield per unit area, however productivity is limited by available land and high input requirements. We determined how much of the food supply of Sydney, Australia, could be produced through UA by synthesising yield data from 13 UA gardens with information on labour and key material inputs and using spatial analyses to assess available land area. We modelled three scenarios with varying proportions of available land used for food production; 25%, 50% or 75% of domestic yard space along with street verges and unused land (e.g. vacant lots). Around 15% of Sydney’s total food supply, or its entire vegetable supply, could be produced through UA under the low range scenario, increasing to 34% under the highest land use scenario. Under the low range scenario, all necessary irrigation water and organic soil amendments could be obtained from local waste streams, though these sources were insufficient to meet the needs of higher range scenarios. Available labour was a limiting factor in all scenarios, with the entire population being insufficient to meet labour needs required to maintain food production under efficiency and labour investment regimes typical of amateur urban gardeners. Establishing a professionalised UA workforce with greater labour efficiency would be required for managing the available land, however this scenario would likely require changes in public attitudes towards use of private land. These social issues, rather than physical limitations, may be the biggest factors preventing cities like Sydney from obtaining a non-trivial proportion of their food supply from UA.