Last year, when the Victorian Liberal Party agreed to radical reform of its preselection mechanism, it was agreed that sitting MPs would be exempt from the new rules for the current term. That left an open question how much immediate effect there would be — how many party members would actually get to use their new right to vote in preselections.

As it’s turned out, the answer is “plenty”. The three federal seats with the most members have all been left vacant by the retirement of their sitting MPs: Petro Georgiou in Kooyong (where preselection takes place next weekend), David Hawker in Wannon, and now Peter Costello in Higgins.

The Higgins preselection has been an on-again, off-again saga. For more than a year now, there have been two semi-declared candidates, former party vice-president Jason Aldworth and former state director Julian Sheezel, both from the Kroger-Costello group. But over the last six months, pretty much everyone concerned — including the two candidates — seemed to have convinced themselves that the contest wouldn’t happen and Costello would stay on.

Even Costello’s closest supporters appear to have been fooled — or if they knew what was coming they did a superb job of hiding the fact.

Not surprisingly, Higgins is good territory for the Kroger-Costello group. The last preselection in the area, for the state seat of Malvern three years ago, was a comfortable win for their candidate, Michael O’Brien. (I’m not going to make repeated disclosures here — just take it that many of the people mentioned are personal friends.)

But that group recently has not been holding together well. Broadly speaking, it is divided between a majority centred around Michael Kroger, his ex-wife Helen and her fellow-senators Michael Ronaldson and Scott Ryan; and a minority surrounding Costello, Sheezel and Senator Mitch Fifield. The former supported Malcolm Turnbull in last year’s ballot, and is backing Aldworth for Higgins; the latter supported Nelson and is backing Sheezel.

Be aware, though, that these are tendencies rather than formed factions; they do not always vote together, powerful individuals can often detach sub-groups on particular issues and if anything the tendency seems to be towards further balkanisation.

The rival Baillieu group now also has a candidate for Higgins, in the shape of IPA director John Roskam. It might be wondered why, if he wants to be in the House of Representatives, he declined to stand in Kooyong, where he lives and where the Baillieu group is much stronger.

But the question answers itself: Roskam is an independent thinker and a Baillieu loyalist was a more logical candidate in a seat where they had strong support. In Higgins, which would have been a long-shot for them anyway, Roskam can be allowed to try his luck — indeed, by playing hard to get he has probably improved his profile.

It’s always been true that a sufficiently high-profile candidate can override the factional dynamics in a seat. With preselection thrown open to a membership of maybe a thousand, the potential for an upset should be much greater. But no-one really knows how the new system will play out, and the next couple of months will be a very interesting time for the Victorian Liberals.