Focused monitoring design, based on information on where and how frequently to monitor contaminants is key to measuring progress towards water quality outcomes. We have developed a model that relies on setting two parameters to predict change in a third: namely, combinations of 1) a percentage change in water quality analytes, 2) the likelihood of detecting a percentage change (viz. statistical power) and 3) the number of samples required over time to confirm a change has occurred. The model has been developed using 20 years of data from ~950 sites for riverine concentrations of nitrate-nitrogen, ammoniacal-nitrogen, total nitrogen, dissolved reactive phosphorus, total phosphorus, E. coli, turbidity, and clarity, and can be applied at any reach in streams outside of national parks and of third order or greater. We present a draft application of this model as an interactive map and explore how monitoring design could be designed to measure whether Government targets of maintaining and improving water quality within 5 or 20 years are achieved. Preliminary analysis of data from monitored sites indicates that a 30% decrease with ≥ 80% power would be detectable in 4-99% of monitored sites (depending on analyte) in 5 and 20 years. To resource this, assuming current operational and capital costs would cost 126 and 442M NZD, respectively or 25.3 and 22.1M NZD per annum.