The Mackerras pendulum was devised by the Australian psephologist Malcolm Mackerras as a way of predicting the outcome of an election contested between two major parties in a Westminster style lower house legislature such as the Australian House of Representatives, which is composed of single-member electorates and which uses a preferential voting system such as a Condorcet method or IRV.
The pendulum works by lining up all of the seats held in Parliament for the government, the opposition and the crossbenches according to the percentage point margin they are held by on a two party preferred basis. This is also known as the swing required for the seat to change hands. Given a uniform swing to the opposition or government parties, the number of seats that change hands can be predicted.
Classification of seats as marginal, fairly safe or safe is applied by the independent Australian Electoral Commission using the following definition: "Where a winning party receives less than 56% of the vote, the seat is classified as 'marginal', 56–60% is classified as 'fairly safe' and more than 60% is considered 'safe'." [1]
This Mackerras pendulum includes new notional margin estimates in Queensland, South Australia and Victoria due to boundary redistributions. [2] The newly created Division of Bonner and Division of Gorton are also included on this pendulum with a predicted margin as no election results yet exist for both divisions.
Members in italics have declared they will not contest their seats at the election, or have lost their party's preselection.
a Although the seats of Bowman and McMillan were Labor wins at the previous election, the redistributions in Queensland and Victoria changed them to notionally marginal Liberal seats.
b Con Sciacca was not contesting Bowman in the 2004 election, but rather transferring to contest Bonner instead.
c Although the seat of Wakefield was won by the Liberals at the previous election, the redistribution in South Australia changed it to be a notionally marginal Labor seat.
d Michael Organ was elected in a (b/e) with a margin of 2.2%