Forecasting the nature and form that the Australian illicit drug market will take by the start of 2018 based on the opinions of police detainees - who engage with illicit drug markets as users and suppliers - provides a unique perspective on likely market fluctuations and influences. Police detainees predicted that by the start of 2018 the most likely changes and greatest impacts will occur within the cannabis and methamphetamine markets, with the heroin market remaining relatively stable. Improvements in cultivation technology and the emergence of new cannabis strains were predicted to impact the cannabis market. An increase in the number of injecting methamphetamine users was predicted to have high impact on the methamphetamine market in terms of health and crime costs to the user and community. Predicting market influences supports government, law enforcement and health sectors to proactively develop strategies to address future challenges in the Australian illicit drug market.