We create the first mergers and acquisition database for Australia using administrative microdata over the period 2003-2020. Mergers are identified through a combination of three methods. First, clusters of employees are tracked between firms in a linked employer-employee database. Second, we exploit capital gain tax data indicating when a merger or demerger via equity swap has occurred. Finally, we use data on share ownership change forms lodged in the event of a merger or takeover. We use this database to assess the impact of mergers on firm performance.