One time Howard government minister and opposition leader Dr Brendan Nelson has announced his intention to immediately retire from Australian parliamentary politics.

Nelson’s imminent departure represents another step in the process the federal Liberal party has had to take to rejuvenate its party room before it can become a viable opponent to Kevin Rudd and the Labor party.

Nelson’s decision to get out is not unlike Peter Costello’s decision to retire, although one very important point of difference is worthy of note.

Whereas Mr Costello will wait until the next general election to take his leave of the political scene, Dr Nelson intends to go now.

This will mean that a by-election will have to be held for Nelson’s seat of Bradield.

In choosing to go sooner rather than later, and in forcing a by-election, Dr Nelson has been able to have one last swipe at Malcolm Turnbull – the man who replaced Nelson as opposition leader on the grounds that Dr Nelson’s opinion poll approval ratings had fallen below 20 percent.

Currently it is Mr Turnbull whose approval ratings are below 20 percent, and there has been pressure on his leadership from within the Liberal party as a result.

By forcing a by-election at this time, Dr Nelson’s retirement may increase the pressure on the current opposition leader.

By-elections have become increasingly volatile phenomena. Because they occur at a much greater rate in seats that tend to be very safe seats, there is always scope for a by-election to embarrass one or other of the major parties and their leaders.

In times gone by, both major parties would nominate candidates to contest a by-election.

Over the last twenty years, however, an interesting behaviour has crept in to the way the major parties now deal with by-elections.

These days there is a tendency for the major party for whom the vacant seat would not be considered safe to strategically absent itself from the contest.

The absenting major party usually justifies its failure to nominate a candidate on the grounds that it does not want to waste its resources.

There is a second motive, however, and this relates to the way in which the absence of a major party candidate along with the tendency for a veritable Caulfield Cup field of independent and small party candidates to nominate affects the chances of the other major party candidate.

In short, the freeing up of a large swag of votes by the failure of one major party to nominate a candidate greatly enhances the chances of the other non-major party candidates to score an upset win.

This is exactly what happened in 2002 in a by-election for the safe Labor seat of Cunningham in NSW.

Thirteen people nominated for the seat, but there was no Liberal party candidate. The Greens’ Michael Organ won a primary vote of 23 percent, but after preferences were distributed, he won a majority of the two party vote and secured the seat from Labor.

Malcolm Turnbull’s greatest fear would be that the Bradfield by-election throws up a result like Cunningham in 2002.

If the Greens or a local independent were to win the seat, this would be a disaster for Turnbull’s leadership. Even a very close result could cause problems for the opposition leader.

On the other hand, a sound Liberal victory would achieve little for Turnbull as most commentators would simply say that Bradfield is a very safe Liberal seat and that a Liberal candidate ought to win it comfortably.

Brendan Nelson, forced from the Liberal leadership by Malcolm Turnbull, has found a way of causing his rival some additional anxiety by bringing on a by-election in a hitherto safe Liberal seat where, over the last few elections, the Greens have been polling strongly.

The danger of an upset result can never be discounted in a by-election. Dr Nelson may yet exact some revenge for the role played by Mr Turnbull in Nelson’s loss of the Liberal leadership.

Dr Nick Economou is a lecturer in politics in the School of Political and Social Inquiry, Monash University.