Western Australia's 1.5 million voters will go back to the polls to elect six senators after the High Court voided the 2013 election result.
The Australian Electoral Commission petitioned the High Court for the election to be declared void after it lost 1375 WA Senate votes during a recount after the September 2013 election.
Justice Kenneth Hayne said on Thursday the missing votes meant he could not determine who was duly elected.
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 said Senator Ludlam and Mr Dropulich were not duly elected because the number of ballot papers lost far exceeded the margin between the candidates.
The federal government was ordered to pay the costs of each of the candidates, and the AEC will return their deposits.
Prime Minister Tony Abbott will effectively set the new election date, but the election writ will be issued by WA Governor Malcolm McCusker.
The AEC declined to comment on possible election dates.
Senator Pratt said the fresh election was an opportunity for WA voters to hold Prime Minister Tony Abbott and state premier Colin Barnett to account for their "broken promises" on the NBN, infrastructure and Medicare.
"We look forward to this election because it will be an opportunity to vote for a Senate that can hold the Abbott government to account for its attacks on working people and working conditions in this country," she told reporters.
Special Minister of State Michael Ronaldson said the AEC needed to ensure such a "failure of process" never occurred again.
"The people of Western Australia, having cast their vote at the last federal election, will rightly be aggrieved that the actions of the Australian Electoral Commission will force them to vote again in a fresh election," Senator Ronaldson said.
The AEC said in a statement the election would be held in accordance with electoral laws.