Arsenic (As) and antimony (Sb) often co-occur in floodplain depositional environments that are contaminated by legacy mining activities. However, the distribution of As and Sb throughout floodplains is not uniform, adding complexity and expense to management or remediation processes. Identifying floodplain morphology predictor variables that help quantify and explain As and Sb spatial distribution on floodplains is useful for management and remediation. We developed As and Sb risk maps estimating concentration and availability at a coastal floodplain wetland impacted by upper-catchment mining. Significant predictors of As and Sb concentrations included i) distance from distributary channel-wetland intersection and ii) elevation. Distance from channel explained 53 % (P < 0.01) and 28 % (P < 0.01), while elevation explained 42 % (P < 0.01) and 47 % (P < 0.01) of the variability in near-total Sb and As respectively. As had a higher extractability than Sb across all tested soil extractions, suggesting that As is more environmentally available. As and Sb dry mass estimates to a depth of 0.1 m scaled to the lower coastal Macleay floodplain ranged from 113–192 tonnes and 14–24 tonnes respectively. Landscape-scale modelling of metalloid distribution, informed by morphology variables, presented here may be a useful framework for the development of risk maps in other regions impacted by contaminated upper-catchment sediments. Display omitted •We examine drivers of As and Sb spatial variability in floodplain wetland sediments.•Wetland elevation and distance from channel help to predict As and Sb distribution.•Integrating these landscape morphology components aids model prediction accuracy.•As is more environmentally available than Sb based on selective soil extractions.