Contestability of Australian federal elections

Using data on federal elections for both the Upper and Lower Houses in Australia and the Herfindahl-Hirschman Index (HHI), we explore whether the contestability/concentration varies across elections. Our results show significant movements in contestability over time. The results also imply a smaller degree of concentration than we would expect to find in a two-party system. To the extent that the Upper House (Senate) - until the 2004 election - is not controlled by the executive, this implies a greater level of political competition, and choice for voters, than that associated with two-party systems.