The presence and consequences of exotic plant species is a major concern for land managers and other stakeholders in the Murray and Murrumbidgee River regions. Particular concerns have been raised in relation to the potential for changes in weed distribution resulting from the relaxation of flow constraints proposed under the Reconnecting River Country program (previously the Constraints Measures Program). This program, run by the New South Wales (NSW) Department of Planning and Environment (DPE), aims to improve wetland and floodplain connectivity through investigating relaxing or removing some of the constraints or physical barriers that impact delivering water for the environment. It focuses on the following areas in the southern-connected Murray Darling Basin (the basin), including: • Hume to Yarrawonga (River Murray) • Yarrawonga to Wakool (River Murray) • Murrumbidgee River As part of this program, NSW DPE contracted Griffith University to conduct an assessment of current weed distributions and consequences and assess potential risks and benefits associated with constraints relaxation. The aims of this project were to: • describe the current invasive weed distribution and consequences in the project areas through a compilation and synthesis of existing knowledge; • evaluate the likelihood and consequences of various flow constraint relaxation options changing invasive weed extent and impacts in the project area; and • develop a risk framework for invasive plant species in relation to each flow scenario. A comprehensive review of published literature and internet resources was conducted in the first stage of this project to address the first aim. This review identified over 80 weed species of concern and described the current invasive weed distribution and consequences in the project area (see Capon et al., 2021). To explore the current distributions of weeds in the project areas, as well as potential changes to these under the inundation scenarios, species distribution models (SDMs) were developed for each catchment (i.e., Murray and Murrumbidgee) under a base case and in relation to each inundation scenario. Seven weed species and two plant functional groups (comprising an additional 38 species) were the focus of this investigation. To build these SDMs, we used species observation data from Atlas of Living Australia and additional data held by the NSW government. Climatic data (i.e. annual rainfall, temperature range), environmental data (i.e., land use, vegetation, and wetland mapping), and inundation metrics were included as predictor variables in the models. The SDMs generated map outputs of the likelihood of the presence/absence of the weed species examined in the Murray and Murrumbidgee project areas under each scenario from which we delineated areas of suitable habitat and highly suitable habitat (i.e. top 20% of suitable habitat). To determine the land uses, vegetation types and wetland classes with most suitable habitat for the weed species considered in each scenario and the changes predicted under these from the base case we conducted a range of spatial data analyses. The results of the SDMs and spatial analyses are summarised below: • Most of the species and functional groups investigated exhibited potential basecase distributions of suitable habitat between 10,000 and 60,000 hectares over the whole project area. The basecase distribution of Phyla (lippia) was much larger, covering approximately 300,000 hectares of potential suitable habitat, which mostly occurred in the Murray catchment. Suitable habitat for Sagittaria (arrowheads) only occurred in the Murray catchment. Salix (willows) had minimal suitable habitat throughout the project area. • Climatic variables were the most important predictors in all SDMs and annual rainfall was the most important predictor in five of seven models. Inundation metrics were moderately important for all taxa, but metrics associated with longer dry periods were particularly important for terrestrial weed taxa, e.g. Marrubium (Horehounds), Lycium (African boxthorn). • Weed hotspots (defined here as areas comprising suitable habitat for four or more modelled weed taxa) occupied less than 1 % of the project area and tended to occur in the vicinity of all the major towns in the project area (Wagga Wagga, Hay, Albury, Echuca, Denliquin and Swan Hill) as well as along the Murrumbidgee Rivers south of Griffith. • Distribution of suitable habitat area for amphibious or aquatic weed species (i.e. species which require flooding for their lifecycle, e.g. Phyla (lippia), Sagittaria (arrowheads) tended to decrease under relaxed constraints scenarios, particularly in the Murray but also in the Murrumbidgee, albeit to a lesser extent. Amphibious species which have thrived in some lowlying habitats under recent reduced flow conditions, appear likely to be ‘drowned out’ by the increased duration, frequency, and permanence of inundation events proposed under constraints relaxation scenarios. • Terrestrial species (i.e. species which do not require flooding for their lifecycle), particularly the widespread Marrubium (horehounds) and Lycium (African boxthorn), exhibited increased potential suitable habitat area under relaxed constraints scenarios in both study catchments. The potential increase in fringing areas (i.e. where moisture is readily available more frequently but where inundation does not occur for longer periods of time) would likely favour the germination and establishment of terrestrial species under a more frequent occurrence of wetter conditions. • Although modelled changes in weed distributions were often substantial between the basecase and flow scenarios, minimal differences in projected species distribution occurred between inundation scenarios. Where differences were notable, Salix (willows) for example, greater potential weed extents were predicted under lower constraint relaxation scenarios, suggesting that the higher flooding conditions resulting from greater constraint relaxation will be unsuitable for this species. • Results of the expert elicitation activities largely aligned with the model findings, although experts generally noted low to moderate confidence in their responses and suggested minimal changes to weed distributions under proposed inundation changes. Model outputs showed varying directions and magnitudes of changes, however, there was little variation between constraint relaxation scenarios for each taxa. • The weed risk assessment framework considered the potential changes in species distribution under each constraint relaxation scenario overall and in land uses, vegetation types, wetland types. Total risk scores for each species were largely consistent between constraint relaxation scenarios with an overall negative score for all scenarios in the Murray River project area corresponding to a slight overall benefit in this region. In the Murrumbidgee, the lowest constraint relaxation scenario (32GL) had an overall positive score, corresponding to a slight overall risk in this region, while the two higher constraint relaxation scenarios had an overall negative score, corresponding to a slight overall benefit in this region.