A statewide byelection in Western Australia looms as a referendum on the Abbott government's first six months, with Labor set to hammer growing fears of changes to Medicare and the introduction of a GP co-payment requirement on patients.

Taxpayers will pay $13 million or more for the unprecedented Senate election, made necessary by a bungled vote count. A clearly livid Abbott government is now weighing up options for a suitable poll date to settle all six seats through a statewide byelection.

It follows the Court of Disputed Returns declaration of the September election count as ''absolutely void''.

It said lost ballot papers made it impossible to say who won the final two Senate spots, thus voiding the entire contest.

''The number of ballot papers lost far exceeded the margin between the candidates at the determinative point in the count,'' the court summary said.

The Liberal Party goes into the election with the most to lose, having secured a better than expected three senators out of the six seats available in September. Strategists predict the government's vote will go down, despite Labor's unpopularity west of the Nullarbor, because of the Coalition's budget plans and its moves to reform Medicare.

The likely loss of its third WA Senate seat would force it to negotiate with a gaggle of as-yet untested minor party rookies in order to pass legislation.

But Labor, the Greens and newcomers such as the Palmer United Party and the Australian Sports Party could also go backwards, given that each had someone elected in either the first or second disputed counts of the September 7 election.

Aware of a rapidly closing window for a snap election on March 29 - an option favoured by some in the cabinet in the hope of ending the pain - the government is also looking at April 5. That is the last available Saturday before school holidays start across the sprawling state.

The rush reflects the government's desire to resolve a long-running distraction and get the election out of the way before delivering an unpopular budget on May 13.

''We certainly don't want to go after that,'' one insider said.

The court's ruling comes after an electoral debacle caused by the loss of 1370 ballot papers by the Australian Electoral Commission.

Government sources conceded they were in entirely uncharted waters because vacancies in the Senate were normally filled by appointment from the same party as a matter of convention.

The WA debacle has raised serious questions about the AEC's processes and its ability to maintain voters' trust.

An independent investigation by former federal police commissioner Mick Keelty was indeterminate but did not explicitly rule out malfeasance.

''The loss of a box of ballot papers with a market value of perhaps $30 could, in the event of the High Court ordering a fresh election, have a $13 million consequence,'' Mr Keelty said.

Special Minister of State Michael Ronaldson is considering when to recommend an election to WA Governor Malcolm McCusker, who must issue the writs.

Mr Ronaldson lamented the AEC's ''failure'' to deliver a proper election result.

The chairman of the joint standing committee on electoral matters, Liberal MP Tony Smith, said the High Court's decision ended five months of uncertainty for WA voters after a ''cavalcade of careless failures by the AEC''. He said the committee would continue its examination of the AEC's conduct and processes ''to ensure that all necessary changes are implemented to prevent a repeat of this, or any other, extraordinary electoral debacle''.