New South Wales (NSW), like Australia overall, has experienced a large decline in crime since 2000, yet little is known about its causes. This study explored this decline through a developmental criminology lens by examining two birth cohorts involving all those born in New South Wales in 1984 and 1994. Comparisons between cohorts showed that, by age 21, the proportion of the population that had come into contact with the criminal justice system had halved (–49%), with the largest declines in vehicle theft (–59%), other property theft (–59%) and drink-driving (–49%). However, there remained a group of ‘chronic’ offenders (those committing 5+ offences) who committed crime at a higher rate and accounted for a larger proportion of offences (77%) than the 1984 cohort of ‘chronic’ offenders (68%). The crime decline in New South Wales would therefore appear to have resulted from a large reduction in the number of young people committing crime for the first time, although there remains a diminishing ‘hard core’ of prolific offenders.