Background: After a 1996 firearm massacre in Tasmania in which 35 people died, Australian governments united to remove semi-automatic and pump-action shotguns and rifles from civilian possession, as a key component of gun law reforms. Objective: To determine whether Australia’s 1996 major gun law reforms were associated with changes in rates of mass firearm homicides, total firearm deaths, firearm homicides and firearm suicides, and whether there were any apparent method substitution effects for total homicides and suicides. Design: Observational study using official statistics. Negative binomial regression analysis of changes in firearm death rates and comparison of trends in pre–post gun law reform firearm-related mass killings. Setting: Australia, 1979–2003. Main outcome measures: Changes in trends of total firearm death rates, mass fatal shooting incidents, rates of firearm homicide, suicide and unintentional firearm deaths, and of total homicides and suicides per 100 000 population. Results: In the 18 years before the gun law reforms, there were 13 mass shootings in Australia, and none in the 10.5 years afterwards. Declines in firearm-related deaths before the law reforms accelerated after the reforms for total firearm deaths (p = 0.04), firearm suicides (p = 0.007) and firearm homicides (p = 0.15), but not for the smallest category of unintentional firearm deaths, which increased. No evidence of substitution effect for suicides or homicides was observed. The rates per 100 000 of total firearm deaths, firearm homicides and firearm suicides all at least doubled their existing rates of decline after the revised gun laws. Conclusions: Australia’s 1996 gun law reforms were followed by more than a decade free of fatal mass shootings, and accelerated declines in firearm deaths, particularly suicides. Total homicide rates followed the same pattern. Removing large numbers of rapid-firing firearms from civilians may be an effective way of reducing mass shootings, firearm homicides and firearm suicides.