Not surprisingly, the majority of the lower house seats in New South Wales are in the Sydney metropolitan area (about 53 out of 93, depending on just where you draw the line). Before looking at where the most interesting ones will be, there are two things that need to be explained about the city’s geography.

First, Sydney is demographically more polarised than Australia’s other big cities: rich and poor areas are less intermingled. Whereas in Melbourne, for example, most of the eastern and south-eastern suburbs are natural marginal seat territory, the class barriers in Sydney are more salient and the scope for seats to change hands is correspondingly less.

In a normal election (which this is not), most of Sydney’s seats are safe for one party or the other; the marginals tend to be quite small in number and widely scattered. The interesting thing about this year is seeing what is usually safe Labor territory become contested.

The second thing (the two are not unrelated, but the connection between them is complex) is that Sydney is geographically more constrained: coastline, harbor, rivers and mountains hem it in, so development proceeds unevenly and along relatively discrete corridors. The Liberal territory north of the harbor is of no real interest this time, so attention will be focused to the west, south-west and south.

Today we’ll look at the western suburbs: the band of 11 seats stretching along the Great Western Highway corridor from Auburn to the Blue Mountains. In 2007 Labor won all of them comfortably, the closest being Londonderry on 6.9%.

That changed last year when the member for Penrith, who was sitting on a margin of 9.2%, resigned following an adverse finding by the independent commission against corruption. The Liberals won the subsequent byelection with an extraordinary 25.7% swing, the largest in NSW history, and new member Stuart Ayres is unlikely to be troubled on Saturday.

Now most of the other 10 are in the firing line. The only seats Labor can be moderately confident about are Auburn (28.7%), at the eastern end of the corridor (it was once held by Jack Lang), and the duo of Mount Druitt (25.4%) and Blacktown (22.4%) — although even Blacktown has been subject to some speculation.

West of Mount Druitt is the more volatile end of the corridor, famous for hosting the federal seat of Lindsay and all that it represents. In addition to Penrith, Londonderry, Mulgoa (11.1%) and Blue Mountains (also 11.1%) are looking like Liberal gains: in the latter two, Labor’s sitting MPs have seen the writing on the wall and are not recontesting.

The other area that must now be considered marginal is the middle section of the corridor between Parramatta and Blacktown — the seats of Riverstone (10.1%), Granville (11.1%), Parramatta (13.7%) and Toongabbie (14.5%). Former premier Nathan Rees in Toongabbie might be able to command something of a sympathy vote, but the others are looking perilous; the members for Riverstone and Parramatta are also among those who have already taken the exit option.

Despite the careless talk about “Howard’s battlers” and the like, most of this region (the Penrith end is a limited exception) is emphatically not swinging territory; it is Labor heartland. The fact that the ALP looks like winning only a minority of its seats speaks volumes for its overall predicament.