The true extent of crime victimisation is unlikely to ever be known, yet new statistical techniques offer a promising method of uncovering the ‘dark figure’ of hidden victimisation. One such technique is multiple systems estimation (MSE), which counts the overlap of victims appearing in different combinations across multiple data sources. Using this technique, it is estimated that the number of human trafficking and slavery victims in Australia in 2015–16 and 2016–17 was between 1,300 and 1,900. This means there are approximately four undetected victims for every victim detected. Quantifying the extent of human trafficking and slavery victimisation enhances our understanding of the problem and provides a sound evidence base for informing policy development, the provision of victim support and law enforcement activities.