The influence that catchment development has on the growth and survival of the Sydney rock oyster, through its effect on the quality and quantity of seston was investigated in Brisbane Water and Lake Macquarie. Developed locations recorded elevated levels of dissolved nitrogen (NH3 and NOx) and chlorophyll a compared to undeveloped locations. Total particulate matter (TPM), particulate inorganic matter (PIM), particulate organic matter (POM), particulate carbon (PC) and particulate nitrogen (PN) were all significantly higher at developed locations in both estuaries. The PC/Chl a ratios were high, indicating that the seston was comprised of detrital matter as opposed to living phytoplankton, yet the C/N ratios were low compared to those expected of estuarine conditions. Sydney rock oysters cultured at the developed locations attained a greater mean shell length and live weight than those cultured at undeveloped locations however higher mortality rates were observed at developed locations. In Brisbane Water, seston quantity measures (TPM, PIM, POM, PC, PN) were positively related to live weight growth rate (GR) while shell length GR was negatively related to salinity. In Lake Macquarie, POM had a positive relationship with oyster live weight GR while shell length GR also had an inverse relationship with salinity. Thus, elevated seston loads associated with low salinities contributed to increased oyster growth at the developed locations where run-off is high. The concentration of the seston was therefore the controlling factor in the growth of oysters cultured in Brisbane Water and Lake Macquarie. The seston quality ratios indicated that the detrital, particulate component of the seston was high-quality food for bivalves, possibly due to associated microbial biomass. In New South Wales (NSW), a high proportion of oyster leases are located in estuaries sourced by developed catchments and the results of this study will be of interest to resource managers and oyster farmers.