Voters in Western Australia will find out on Tuesday whether they will go to a fresh Senate election which could determine the fate of the Abbott government's agenda.
High Court justice Kenneth Hayne will decide on a petition brought by the Australian Electoral Commission to have the election of six WA senators in 2013 declared void.
The AEC lost 1370 votes in a recount of the WA Senate election.
An independent inquiry by former police chief Mick Keelty was inconclusive about the fate of the ballot papers, but called for a major overhaul of the AEC's processes.
Three Liberals and one Labor candidate were declared winners of the first four of six seats.
The initial count of the final two seats gave wins to Zhenya "Dio" Wang of the Palmer United Party (PUP) and Labor's Louise Pratt.
After a recount, the candidates narrowly elected to the fifth and sixth Senate positions were the Australian Sports Party's Wayne Dropulich and Australian Greens senator Scott Ludlam.
ALP national secretary George Wright argued the High Court sitting as the Court of Disputed Returns should recognise it was the intention of WA voters to re-elect Senator Pratt.
He said the court should use its powers to look at the disputed ballot papers, consider the records of the lost votes and declare the correct candidates elected.
The Liberals and Palmer United Party also argued for the initial result to be upheld.
However, the Greens and Mr Dropulich said the recount result should stand.
The Abbott government will need the support of the new senate, which starts on July 1, to get its carbon and mining tax repeal bills passed as well as other legislation.
As it stands, from July 1 the government will hold 33 seats in the senate, with Labor holding 26, the Greens nine, PUP three, plus one each for the Nick Xenophon Group, the Democratic Labor Party, Liberal Democratic Party, Family First and Australian Motoring Enthusiast Party.
Justice Hayne told the court declaring the election void and forcing a new ballot would be expensive, politically difficult and take some effort and time to organise.
The judge also has the option of referring the issue to the full bench of the High Court or finding an alternative way of determining the result.