The looming by-election in Farrer is shaping as a four-cornered contest that could reveal how vulnerable the Coalition has become.

The by-election for Sussan Ley’s regional electorate of Farrer is likely to come down to a four-cornered contest between One Nation, the Liberals, the National Party and an independent.

Of those, the Nationals have the worst prospects and may opt out of a fight that would probably demonstrate their increasing electoral weakness.

Labor, which would have no chance of winning, will probably save its money and watch from the sidelines – while cheering on and providing votes to the independent.

From 1984 to 2001 Farrer was held for the National Party by Tim Fischer, who was deputy Prime Minister under John Howard. Before that it had been held by Liberals. On Fischer’s resignation the seat was contested by both the Liberal and National Parties and was won by Sussan Ley – narrowly defeating the National Party candidate by just over 200 votes.

Under the Coalition agreement, the Nationals were unable to contest the seat again while it was held by Ley, but now they are free to do so.

But will they risk it? That is likely to depend on whether they see One Nation as taking votes from the Nationals or the Liberals. The national polls published over the past few months don’t provide an answer. They measure the would-be vote for the Coalition, not for its constituent parties. Not least, this is because on an Australia-wide basis, the National Party vote is too small to be separated out.

Farrer will tell us whether One Nation is hurting the Liberals or the Nationals the most, or whether those rebellious former Coalition voters are not differentiating between them.

It is important for both the Liberals and the Nationals that they know. That will help them work out how they should counter a threat that without exaggeration can be described as existential.

Voting in Farrer could help determine whether one or both the Coalition members need to make policy changes that will move them further to the right, so that they compete directly with One Nation.

Or, in the case of the Liberals, whether they can concentrate instead on trying to regain the millions of voters they have alienated in the cities.

Meanwhile the by-election will give voters the opportunity to turn their backs on all three of the right-wing parties and send yet another independent into the House of Representatives.

At the last election, Sussan Ley’s main challenger was an independent. Ley won just over 43 percent of the primary votes, Labor won 15 per cent, while independent Michelle Milthorpe had 20 per cent. After preferences, Ley defeated Milthorpe and finished with just over 56 per cent of the vote. In the previous election she won over 61 per cent of the vote after preferences. An ominous trend.

Milthorpe is just one of a number of independents in country and provincial electorates seriously challenging sitting Liberal and National Party MPs. Like some others, she was supported at the last election by Climate 200. And like others with that funding and support, she resists being labelled as a Teal. Actually, everyone in that camp dislikes being publicly labelled as a Teal. But that is the association – and it probably didn’t hurt her in the last election.

If – when – it comes to distributing preferences in Farrer it will be fascinating to see what happens to the preferences of the Liberal or National candidate who is eliminated during the count – assuming both contest the by-election. What proportion will go to One Nation if that party is ahead at that point in the preference count?

We know that One Nation preferences do not go as solidly to the Coalition parties as, say Green preferences go to Labor – in recent elections roughly 70 per cent from One Nation to the Coalition, as against more than 80 per cent of Greens to Labor. At the by-election would the Coalition preferences follow the relevant how-to-vote card? Presumably the Coalition parties would preference One Nation above the independent – unless they perceive the threat from One Nation as more threatening.