This paper estimates the impact of fertility on mothers' labour supply in Australia, using exogenous variation in family size generated by twin births and the gender mix of siblings. Results show that having more than one child decreases mothers' labour market participation by 12 percentage points and hours worked by around four hours per week. Having more than two children reduces labour market participation by 12 percentage points and hours worked by three hours a week. Compared with other countries, the effects for Australia are large.