There are a handful of seats in the House of Representatives where the Greens are within spitting distance of winning the seat. In nearly all of them, however, the true margin is hidden by the AEC’s methods of measuring marginality. Margins are calculated by referring to the two-candidate-preferred vote, but this doesn’t take into account the possibility that a candidate in third place may overtake the second-place candidate and be in a winnable position.
Greens marginal seats run into what psephologists call “non-monotonicity”, whereby one group’s preferences are not reciprocated. In monotonic system, a candidate cannot be harmed by receiving an extra vote. In recent federal elections, Liberal preferences in strong Greens seats have been directed to the Greens over Labor, putting the seat in a position where a Green in third place might win against Labor in a 2CP count, on Liberal preferences, but the preferences are not reciprocated, with Greens voters preferencing Labor over Liberal. Thus a seat may be registered as a 19.5% margin for Labor, as in the case of Sydney, even though the Labor vs Greens margin would be much smaller. Even if the result isn’t reversed by a change in the order of elmination, it’s possible that a seat can shift from “Very Safe” to “Marginal”.
At the 2007 election, only one Greens candidate managed to come second after preferences, in the seat of Melbourne, where the Greens overtook the Liberals on preferences from minor candidates and registered over 45% of the two-party-preferred vote, making the seat marginal. But there are a number of seats that fit the criteria of Greens marginals but don’t appear as such on the pendulum. These criteria are:
- A reasonably high Greens vote (above 15%).
- A large gap between the two major parties, with the weaker of the two (usually the Liberal) very low (usually below 30%).
The Greens managed to attract 82.5% of Liberal preferences in Melbourne at the 2007 election. In order to determine the next five most marginal Greens seats, I translated this proportion of Liberal votes to the Greens, to determine a two-party-preferred vote between Labor and Greens. I excluded all seats where the margin between Liberal and Greens is greater than the Labor-Greens margin. For example, in the case of Grayndler, I calculated a 60-40 margin, and the Liberals beat the Greens by 2%. I assume that if the Greens were to gain 10% of the 2PP vote they would manage to overtake the Liberals.
Anyway, here is my calculations.
- Melbourne – 54.7-45.3
- Sydney – 55-45
- Grayndler – 60-40
- Cunningham – 61-39
- Denison – 61-39
- Batman – 62-38