The High Court of Australia, sitting as the Court of Disputed Returns, has voided the result of the Western Australian Senate election held last year.
The Australian Electoral Commission petitioned the Court of Disputed Returns for the election to be declared void, after it lost 1,375 votes during a recount after the September 2013 election.
Justice Kenneth Hayne said he could not determine who was duly elected and nullified the result on Thursday.
The initial count declared the Liberals and Labor winners of the first four of six seats.
The final two seats went to Zhenya Dio Wang of the Palmer United Party and Labor's Senator Louise Pratt.
But the recount narrowly gave the final two seats to the Australian Sports Party's Wayne Dropulich and the Greens' Senator Scott Ludlam.
Justice Hayne ruled on Tuesday that Ludlam and Dropulich were not duly elected because the number of ballot papers lost far exceeded the margin between the candidates.
Hayne ordered that the Commonwealth pay the costs of each of the candidates, and that the AEC return their deposits.
WA Governor Malcolm McCusker will be responsible for issuing the writ for a fresh election.