Now that we have the final voting figures for the City of Brisbane, it was possible to update the analysis of preference flows and the impact of optional preferential voting (OPV) that I published prior to the election.
Immediately after the election, it became clear that there had been an increase in the proportion of Greens preferences flowing to Labor and vice versa at the 2024 election, compared to the 2020 council election.
For this blog post I am comparing the 3CP figures in wards where the final three candidates were LNP, Greens and Labor (which was in 24 out of 26 wards in 2024) to the two-candidate-preferred figure to determine the preference flows.
This table compares preference flows from Greens to Labor and vice versa at the 2020 Queensland state election (using compulsory preferential voting) and at the 2020 and 2024 Brisbane City council elections (using optional preferential voting).
Voter group
Prefs to ALP/GRN
Prefs to LNP
Exhausted
Greens prefs (CPV)
80.1
19.9
Greens prefs (OPV 2020)
47.0
10.1
42.9
Greens prefs (OPV 2024)
53.6
11.7
34.8
Labor prefs (CPV)
74.6
25.4
Labor prefs (OPV 2020)
41.3
12.2
46.5
Labor prefs (OPV 2024)
45.6
16.3
38.0
The proportion of preferences exhausting dropped significantly in both directions. The proportion of the remaining preferences remained roughly the same, so the LNP did also gain preferences. But with most preferences flowing to Labor or Greens, a lower exhaustion rate helps both parties in contests against the LNP.
For this next chart, it just shows wards or state seats in the Brisbane area where Labor and the Greens made the top two, and it shows how preferences from the Greens on the 3CP split.
Since there are no exhausted preferences for federal elections, the Labor and LNP preference flows add up to 100% and thus they are all on a line of X = 1 – Y.
It’s very obvious here that the rate of preferences flowing in 2024 was higher than in the 2020 council election, with the exhaust rate dropping by about 10%. Thus the 2024 results were a bit closer to what you’d get under compulsory preferential voting.
As I did with the 2020 results, I have then estimated how different the margins would have looked under compulsory preferential voting.
For this analysis I’ve just looked at the 24 wards which ended up as a contest between the LNP and either Labor or Greens. It doesn’t cover Tennyson (won by an independent) or Moorooka (where Labor won and the Greens came second).
I think Labor would have won Northgate and Wynnum-Manly under CPV.
Interestingly I don’t think the Greens would have picked up any extra wards under CPV, but the margin in Coorparoo, Walter Taylor and Enoggera would have been much closer, with the Greens over 49% in all three.
To be honest, I had expected the difference between OPV and CPV to be bigger, with such an even split in support between Labor and the Greens leading to a lot of exhaustion and thus the potential for more wards to produce a different outcome under CPV. The limited number of wards where the LNP would have lost under CPV partly reflects how few wards ended up being that close. The LNP only won with a margin of less than 51% in just one ward: Northgate.
But the first chart makes clear that the gap between OPV and CPV in 2020 is not as wide as the gap between CPV and the 2024 OPV results.
While there’s just two wards that would have flipped, if you look at the margins it’s not insignificant. CPV would have reduced the LNP’s margin for lord mayor from 6.4% to 4.3%. The LNP would need to lose five wards to no longer hold a council majority. Under the actual result, a uniform swing of 2.3% would have been needed for the LNP to lose five wards. Under CPV, that swing would have just needed to be 0.4%.
The choice of system does make a difference, with the party in the lead winning more often under optional preferential voting, while two trailing parties can exchange preferences more efficiently under compulsory preferential voting. But while some compare OPV to first-past-the-post, it’s not the same thing. Preferencing behaviour can change, and that can change the outcomes.