At the time of writing, we don't have the breakdown of BTL votes from the WA election, so I'm taking this opportunity to see how we'd expect to see a "below-the-line for Pratt" movement, by looking at some results from 2013. The most interesting thing to me is that Pratt was already very popular BTL relative to Bullock last September.
For each of the major parties and Greens (and all the other parties, but I haven't bothered posting them here), I calculated the ratio of BTL votes for the first candidate on the ticket to the votes for each subsequent candidate. For example, the first entry in the numbers below reads "Brown to Bilyk, 5.5": in Tasmania, Carol Brown received 5.5 times as many BTL votes as Catryna Bilyk.
I'm looking at the sample of voters who supported a party and chose to vote below the line; perhaps this was because they didn't like their party's group voting ticket, or perhaps they didn't like the order of candidates their party had chosen. I'm interested in this latter case.
The ratio of BTL votes between first and second (or third, fourth, ...) candidates is not a perfect way to capture supporter dissatisfaction with the leading candidate – if the GVT in a state is unpopular, then that would lead to more of that party's supporters voting below the line, and they would likely be giving the top candidate their first preference, thus inflating the ratio that I calculate. The natural way to capture dissatisfaction with the GVT is to just look at the percentage of votes for the party that were below the line, but this is problematic for the inter-state comparisons that I would like to make – ballot paper lengths and ticket preferences can be significantly different between states, leading to large relative changes in BTL voting rates.
With those caveats out of the way, here are the ratios of a party's lead candidate to subsequent candidates, in order of increasing relative popularity of a non-lead candidate.
Tas: Brown to Bilyk, 5.5; Brown to Thorp 1.8, Brown to Dowling 4.3
NT: Peris to Foley, 2.1
WA: Bullock to Pratt, 2.4; Bullock to Foster, 10.7; Bullock to Ali, 13.8
Vic: Marshall to Collins, 5.7; Marshall to Tillem, 29.0; Marshall to Psaila, 36.5; Marshall to Larkins, 41.4; Marshall to Mileto, 21.1
Qld: Ketter to Moore, 5.7; Ketter to Furner, 21.7; Ketter to Boyd, 9.3
NSW: Carr to Cameron, 8.1; Carr to Stephens, 25.5; Carr to Kolomeitz, 134.2; Carr to Nelmes, 121.1; Carr to Chhibber, 35.0
ACT: Lundy to Sant, 19.0.
Liberals or some sort of LNP:
ACT: Seselja to Nash, 3.5
Tas: Colbeck to Bushby, 4.2; Colbeck to Chandler, 8.9; Colbeck to Courtney, 8.7
Vic: Fifield to Ryan, 8.4; Fifield to Kroger, 4.2; Fifield to Corboy, 6.9
NSW: Payne to Williams, 9.0; Payne to Sinodinos, 4.5; Payne to Hay, 24.0; Payne to C Cameron, 21.6; Payne to A Cameron, 11.0
SA: Bernardi to Birmingham, 5.5; Bernardi to Webb, 16.1; Bernardi to Burgess, 19.8; Bernardi to Cochrane, 67.7; Bernardi to Weaver, 56.1
NT: Scullion to Falzdeen, 6.0
Qld: MacDonald to McGrath, 20.5; MacDonald to Canavan, 31.9; MacDonald to Goodwin, 19.7; MacDonald to Craig, 25.4; MacDonald to Stoker, 13.3
WA: Johnston to Cash, 14.1; Johnston to Reynolds, 15.2; Johnston to Brockman, 29.7; Johnston to Thomas, 22.3; Johnston to Oughton, 14.9
NT: Williams to Brand, 6.5
Tas: Whish-Wilson to Burnet, 8.3; Whish-Wilson to Ann, 24.6
WA: Ludlam to Davis, 10.1; Ludlam to Duncan, 38.2
Qld: Stone to Bayley, 10.9; Stone to Yeaman, 47.5
ACT: Sheikh to Esguerra, 22.0
NSW: Faehrmann to Ryan, 58.1; Faehrmann to Blatchford, 36.9; Faehrmann to Ho, 38.4; Faehrmann to Findley, 53.5; Faehrmann to Spies-Butcher, 45.1
Vic: Rice to McCarthy, 41.7; Rice to Truong, 55.1; Rice to Christoe, 124.7; Rice to Sekhon, 132.7; Rice to Humphreys, 40.3
SA: Hanson-Young to Mortier, 74.9; Hanson-Young to Carey, 66.8
A few obvious things spring out to me. As mentioned earlier, Pratt was already popular with BTL Labor voters compared to Bullock. (She also received more BTL votes relative to Labor ticket votes than any other non-lead Labor Senate candidate outside the Tasmania and the ACT.) Lin Thorp, formerly a minister at state level in Tasmania, was the most popular non-lead candidate by this metric. On the conservative side, both Arthur Sinodinos and Helen Kroger were high-profile candidates who appear to attract a bit of a personal vote. Greens voters seem pretty happy with their lead candidates. On a lighter note, there's more than a hint that voters disproportionately give their first preference to the last candidate in a large group.
Anyway, the little sociology experiment on WA Labor voters will be looking at the Bullock-Pratt ratio as it is counted in the coming weeks; if there was a decent swing against Bullock to Pratt, then we should see the ratio of their BTL votes fall from 2.4 to something lower. My guess is that it will fall a little below 2; I'd put the under-over at around 1.9. (Update: A few hours after posting, and I see that the first couple of hundred BTL's show Pratt getting more votes than Bullock! At this stage it looks like I'd have been closer with 0.9 rather than 1.9.)