Stormwater Quality Offsets Implementation Plan
NCC supported Logan City Council to evaluate and refine its stormwater quality offsets scheme by identifying cost-effective pollutant reduction projects, informing offset pricing, and providing recommendations to support long-term scheme implementation and environmental outcomes.
Urban growth and development can place increasing pressure on waterways through stormwater runoff and associated pollutant loads. Logan City Council engaged NCC to undertake an economic evaluation of its proposed stormwater quality offsets scheme and provide evidence-based recommendations to support implementation.
The project assessed a range of potential offset activities designed to reduce key stormwater pollutants, including total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) across urban and rural catchments. The objective was to identify projects capable of delivering the greatest environmental benefit while providing value for money and supporting the scheme’s long-term viability.
NCC applied a cost-effectiveness framework to evaluate offset options over their full lifecycle, incorporating establishment, operational and replacement costs. Sensitivity analysis using Monte Carlo simulations was undertaken to account for uncertainty in both project costs and pollutant reduction performance. A multi-criteria analysis was also used to assess broader social, environmental and regulatory considerations, providing a comprehensive basis for project prioritisation.
The analysis identified channel naturalisation and bioretention projects as highly cost-effective options for reducing urban pollutant loads, while rural erosion hotspot stabilisation projects demonstrated strong performance in reducing nitrogen and phosphorus loads. The study also examined offset supply and demand across catchments, identifying location-specific constraints that could affect the practical delivery of offset outcomes.
In addition, NCC assessed the implications of offset pricing for development contributions, including estimated costs for low-density residential development. The findings provided insights into potential housing affordability impacts and highlighted opportunities for developers to pursue on-site mitigation measures where cost-effective.
The project delivered practical recommendations to support implementation of the offsets scheme, including optimisation of project selection, stakeholder engagement on pricing structures, and establishment of a robust monitoring framework. These findings provide Logan City Council with a stronger evidence base for managing stormwater impacts and improving water quality outcomes across the region.
