Intensive dairying can impair water quality. Although much science exists on what and how to mitigate contaminant losses from land to water, few data exist on the success of implementing best management practices (BMPs). Five dairy-dominated catchments across New Zealand were studied over 20-years (2001 to 2021). The first period, from 2001 to 2010, involved detailed extension advice to farmers consisting of workshops, water quality (including dissolved reactive phosphorus, total phosphorus, ammoniacal-nitrogen, nitrate-nitrite-nitrogen [NNN], sediment and E. coli) and quantity sampling, farm practice surveys, and investigations to find and solve contaminant losses from land to water. The second period (2011-2021) was termed post-extension where only farm practice surveys and water quality monitoring continued. During the extension period, concentrations of most contaminants (excluding NNN) decreased. Step changes, as sudden decreases in contaminant concentrations, were correlated with on-farm practice change. This included a transition from direct discharge of farm dairy shed effluent from waterways to land application, capture of effluent from off-paddock facilities, and excluding stock from streams, resulting in in-stream reductions in phosphorus, sediment, and E. coli concentrations. Transitioning from flood to spray irrigation, while improving water use efficiency, caused an increase in NNN concentrations. Improvements in in-stream concentrations achieved in the extension phase were maintained or improved in 24 of 30 (80%) contaminant-by-catchment combinations. These data indicate that the uptake of BMPs continued to have a positive effect on in-stream concentrations, alongside other mitigation actions such as riparian restoration and less P fertiliser use. Extension advice and practice change have helped to improve water quality. However, some 2020 national water quality policy targets were not being met thus more action is still required for further reductions. Actions could include new cost-effective BMPs, collective catchment action, deintensification or diversification of land use, or setting more achievable targets in productive pastoral landscapes.