It’s unusual to conduct a redistribution in an election year, but even as political parties gear up for this year’s federal poll, the Australian Electoral Commission has begun the process of redrawing federal boundaries in Victoria. A quirk of timing (explained by Antony Green) means the redistribution has to take place this year, even though the new boundaries won’t come into effect in time for this election.

That might mean that the political parties approach the task with some added detachment, not knowing just which sitting members will ultimately suffer or benefit. More likely, though, it just means they’ll be distracted by other things, so their submissions will contain even more errors and non-sequiturs than usual.

Today the AEC will release the enrolment statistics and projections on which the redistribution will be based (they should be available for download here. All new electorates must have current enrolments within 10% either way of the average, and projected enrolment in 2013 within 3.5% of the projected average.

They will reveal the usual imbalances of growth; the city is gaining population at the expense of the country, provincial towns are growing at the expense of rural areas, and the outer suburbs are growing at the expense of middle and (to a lesser extent) inner suburbs. But there’s also a more specific pattern.

Melbourne’s mortgage belt is growing more to the north and west — the traditionally safe Labor area — than to the south and east, which are traditionally more marginal. On current enrolments, the three most populous seats by a wide margin are in the northern and western suburbs: Gorton, Lalor and McEwen (McEwen is partly rural, but its suburban component is where the growth is). Of the eight smallest seats, six are south of the Yarra.

Collectively, the 19 seats north of the Yarra and the Great Divide are about a third of a seat in excess of the required average enrolment, and that will only get worse with the projected figures. That means at least one of the seats south of the Yarra — most probably Casey — will have to shift substantially north to soak up the extra voters.

It looks as if it should be possible to do this without having to actually create or abolish a seat, something the commissioners generally avoid where possible. It’s made easier by the fact that population growth in the outer suburbs is matched by relative decline on both sides, in the bush and in the established suburbs, so outer suburban seats can contract where needed.

Moreover, there are few geographical constraints to the north and west of Melbourne that prevent seats from moving around; much of the growth has taken place on what is essentially a featureless plain (unlike New South Wales and Queensland, where the growth corridors tend to be more hemmed in by natural boundaries). The most difficult thing will probably be finding additional voters to make up the numbers in the more remote rural seats such as Mallee and Murray.

Politically, it’s impossible to say just who will benefit from the changes. The devil is in the detail, and two redistribution plans that look very similar in outline could have quite different effects depending on just where the boundaries are drawn.

But whoever wins this year in marginal seats such as McEwen and Deakin could find that the boundary commissioners will have a big influence on their tenure.