MELBOURNE'S population boom is forcing the redrawing of electoral boundaries, writes PETER HUNT

Metropolitan Melbourne's massive population growth threatens to swallow up at least one regional seat and jeopardises the Government's control of the Upper House.

Outer Melbourne electorates such as Yan Yean are bursting at the seams with 55,632 voters.

At the other extreme is Agriculture Minister Peter Walsh's electorate of Swan Hill, which has the least number of voters in the state, just 32,845.

The law demands the Electoral Boundaries Commission redresses this imbalance by redrawing electorate boundaries, bringing the state's 88 Lower House seats closer to the 40,674 voter state average.

The commission won't start its review until November next year, but the state's four main political parties are already signing up new members and campaigning in electorates they know will be pulled into marginal Lower House seats or crucial Upper House regions.

The last time the boundaries were redrawn was in 2002. Greater Melbourne's population has grown by more than 600,000 since then. Growth in regional Victoria has been largely restricted to electorates within two hours drive of Melbourne, including key marginal electorates surrounding Bendigo, Ballarat and Macedon.

In contrast, the Victorian Electoral Commission's 2011 annual report shows voter numbers in the National Party-held seats of Mildura, Swan Hill, Rodney, Lowan, Shepparton and Benalla have grown marginally or declined.

Ultimately, the commission must decide how it will bleed Melbourne's outer suburban electorates to bring voter numbers down to the state average and build in room for further growth.

Most of the housing boom electorates - Yan Yean, Yuroke, Macedon, Keilor, Altona, Tarneit, Cranbourne and Narre Warren South - are held by Labor.

Political analysts believe the commission will siphon the excess voters into a new outer-metro electorate in northwest Melbourne.

Creating a new seat means abolishing another, with the most likely contenders being the marginal Labor-held seat of Ripon or one of the National Party-held seats of Rodney or Swan Hill.

But the redistribution doesn't end there.

In most cases the commission will also have to carve chunks off other outer-Melbourne electorates and redistribute voters into neighbouring peri-urban or regional electorates.

This means pushing more urban voters into Victoria's three Upper House regions, which each elect five members to Parliament.

The redistribution threatens to oust the Liberals' northern Victoria region member Donna Petrovich, who scraped into Parliament on the back of Green preferences.

Ms Petrovich went down to the wire in a complex distribution of preferences that put her just 1689 votes ahead of the Country Alliance candidate in a count of 389,673 votes.

The Coalition is worried, given they hold just 21 seats in Victoria's 40-seat Upper House. Liberal Party strategists have already launched a membership drive among northern Melbourne's Indian and Chinese communities.

The redistribution could also cause problems for the Coalition in the western Victoria region, if the commission pours more Labor voters into some of the 11 Lower House seats on which it is founded.

The Nationals' David O'Brien will have to battle the Greens and Labor for the fifth spot in 2014.

The Labor-held Lower House seat of Melton sits in the Upper House western Victorian region, which may explain why Coalition members are taking a keen interest in Mantle Mining's brown coal exploration works at Bacchus Marsh.

Whatever the final outcome, next year's electoral redistribution will send ripples through to the far corners of the state.