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 crossbench according to the percentage point margin they are held by on a two-party or two-candidate basis.
Given a uniform swing to the opposition or government parties, the number of seats that change hands can be predicted.
"[1] Based on the 2013 post-election pendulum for the Australian federal election, this pendulum has been updated to include new notional margin estimates due to redistributions in New South Wales, Western Australia and the Australian Capital Territory.
[2][3] Whilst every federal election after 1961 has been won by those that also won the majority of federal seats in New South Wales, unusually nearly half of all marginal government seats are in New South Wales at this election of which nearly half are all in Western Sydney and the other half all in rural and regional areas, and with no more than a few seats each in every other state.