Both parties have campaigned hard in the electorates – particularly in Braddon and Longman where the seats are held by Labor on the tightest of margins – but there are broader national implications than simply who wins each seat.
At stake is the balance of power in the Lower House, the leadership of the major political parties, and the emergence of campaign strategies ahead of the national election (which Prime Minister Malcolm Turnbull said on Friday wouldn't happen until 2019).
What's at stake for Labor?
The by-elections are being described as Labor’s to lose. Four of the five seats are being re-contested by the Labor MPs who were forced to resign due to the dual citizenship saga.
If Labor loses Braddon (Tasmania) or Longman (Queensland), it would be the first time a federal opposition has lost a seat to the government in a by-election in nearly a century. Braddon itself is on a margin of 2.2 per cent and Longman sits on a wafer-thin margin of 0.8 per cent.
The results are expected to feed the speculation over Labor’s leadership. In the final days of campaigning, Shadow Minister for Infrastructure Anthony Albanese has continued to dismiss suggestions he is interested in leader Bill Shorten’s job.
Polling indicates Mayo (South Australia) is expected to be retained by crossbencher Rebekha Sharkie. The Centre Alliance MP won 34.9 per cent of the vote at the last election, and 55 per cent of the two-party-preferred vote.
That leaves Labor in the position of hoping for the best case scenario of maintaining the status quo and holding onto the seats of Braddon and Longman.
Political analyst Chris Salisbury from the University of Queensland said the Liberal Party simply picking up just one seat would be enough to threaten Bill Shorten.
It “could bring some conjecture as to his hold on the Labor leadership [and] provide something more of a buffer to Malcolm Turnbull’s Coalition in Parliament, with a couple of extra seats at their disposal, and also [contribute to] some shoring up of his leadership.”
What is at stake for the Liberal Party?
The Liberals are hoping to pick up any seats from Labor to increase their one-seat majority in the Lower House.
Such a coup would add momentum heading into the national election and could prompt Malcolm Turnbull to announce a snap election to seize an advantage (even though he said he wouldn't).
Dr Salisbury said Longman’s history suggests the Liberal party is in with a real chance of winning back the seat it last held in 2013.
Since the seat was created in 1996, Labor has only won the seat twice in the last eight contests.
“This by-election perhaps gives an early chance for voters to revert to what has historically been a sort of blue seat,” Dr Salisbury said.
Winning the electorate of Braddon in Tasmania’s north-west will be a litmus test for the major parties' signature policies ahead of the next federal poll. It covers around a third of Tasmania and is an historically conservative electorate that is home to large forestry and manufacturing industries.
Polls suggest the seat is sitting on a knife edge after changing hands five times over the past 20 years.
Labor MP Justine Keay is going up against the same man she snatched the seat from with a small margin of 2.2 per cent in the 2016 election, former Liberal MP Brett Whiteley.
Another candidate to watch is independent Craig Garland who has been a professional fisherman on the Apple Isle almost his entire life. He ran in the March state election and managed to get almost 2,000 primary votes despite running on a $780 campaign budget. He is preferencing Labor and the Greens.
Longman is a sprawling electorate of about 110,000 voters just north of Brisbane on the Sunshine Coast and faces some serious economic challenges and pressures.
The unemployment rate trends well above and median incomes about 20 per cent below than the national averages. Most workers are in trades and clerical jobs and cost of living is their concern.
Susan Lamb is recontesting the seat and facing former LNP state Newman government MP “Big” Trev Ruthenberg; a former RAAF corporal, lay head of the Lutheran Church Australia and recently a community liaison for a property developer.
Each candidate is expected to get just above 30 per cent of the primary vote, so a win depends on preferences from not just one, but a number of minor parties.
There is a particular focus on Pauline Hanson’s One Nation which pollsters say could get 15 per cent of the vote.
Mayo (South Australia)
The seat extending from the Adelaide Hills to Kangaroo Island was long held by the Liberal Party, first by Alexander Downer. His daughter, Georgina, is being fielded as the Liberals candidate against incumbent Centre Alliance MP Rebekha Sharkie.
She won the seat from Liberal MP Jamie Briggs in 2016 on a margin of 5 per cent.
Health, funding for the ABC, and education have been the key local issues of the campaign.
Perth, Fremantle (Western Australia)
Labor is expected to retain the seats in Perth and Fremantle as the Liberals are not fielding candidates in the electorates.
The only sitting MP unaffected by dual citizenship issues, Tim Hammond won the seat in the central suburbs of Perth in 2016 on a margin of 3.3 percent. Former Kevin Rudd staffer and WA Labor secretary, Patrick Gorman, is expected to win this time around.
Fremantle in the south-eastern suburbs of Perth has been a Labor seat for more than 80 years. One of the dual citizenship MPs Josh Wilson is expected to win - he first won in the 2016 election on a margin of 7.5 per cent.
Roads and GST have been key local issues during the campaign.
Which wildcard candidates could win?
With such tight margins in Longman and Braddon, preference flows are expected to be crucial in deciding which major party wins the seats.
In Longman, votes are expected to flow to the Liberal party from preferences of One Nation’s candidate, Matthew Stephen.
Dr Salisbury said the popularity of the One Nation vote in the region should not be underestimated.
“Pre-polling and postal voting tend to favour the Coalition slightly, ahead of Labor. So it might spell some trouble. By I think the voters are.
“It is very predictable because we have the presence in Longman of One Nation, which polled almost 10 per cent at the last election.
In Braddon, the preference flows are expected to favour Labour. Locally popular Independent candidate Craig Garland scooped up 2,000 votes in the March state election.
The fisherman, who is known also by the nickname ‘Garbo’, describes himself as an “anti-politician” who provides Braddon voters with an alternative from the major parties.
Dr Bonham said predicting who will win the seat is not straightforward. “At the last election, nearly two-thirds of minor party preferences went to Labor. And at this election, it is very likely that the same thing will happen, more or less.
“The prominent independent Craig Garland is following a very similar niche to the Recreational Fisher’s party at the last election. And his preferences are quite likely to favour Labor’s votes.”
Over 68,000 people had cast early votes as of Friday afternoon: more than 22,000 in Longman and more than 8,000 in Braddon.
Additional reporting: Marija Zivic and Stefan Armbruster.
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