Ketamine is an anaesthetic drug with dissociative effects and is used in hospitals and for emergency treatment. Over the past four years, there has been increasing interest in the non-medical (recreational) use of ketamine in Australia. This study aimed to interrogate patterns in ketamine loads found in wastewater in Australia between December 2020 and April 2025. Daily influent wastewater samples were collected from up to 50 wastewater treatment plants over a week across state and territory capital city and regional sites every 4 months from all states and territories of Australia covering on average 53% of the total population. Wastewater was concentrated using solid phase extraction followed by quantitative analysis using liquid chromatography tandem mass spectrometry to determine concentrations of the metabolite norketamine, with final concentrations flow and population normalised to units of excreted mg/day/1000 people. Norketamine excreted mass loads were compared with regionality and socioeconomic status.

Considering the intended application, substantially higher mass loads of norketamine on weekends compared to weekdays would suggest the non-medical use of ketamine at weekends. Nationally, norketamine mass loads were higher for sites located in major city areas compared to those in ‘inner’ and ‘outer’ regional Australia. In addition, sites with higher socioeconomic status had higher mass loads of norketamine. Overall, norketamine mass loads have been increasing in Australia from 2022 to 2025, particularly in sites with higher socioeconomic status. This study demonstrates the value of ongoing wastewater monitoring, revealing both within-week and long-term shifts in ketamine excreted loads in Australia.