Tony Abbott has shifted his threat of a double dissolution election, claiming he would now contemplate the option if the new Senate, which sits from July, does not allow the government to repeal the carbon and mining taxes.

“If Labor doesn’t see the light in the next few months there is a new Senate coming in July and I am confident they will accept the government’s mandate and if not there are constitutional options open to us,” the prime minister said in an interview with the ABC to mark the government’s first 100 days in office.

But businessman Clive Palmer’s Palmer United Party will hold crucial balance of power votes in the new Senate and he told Guardian Australia it was “wrong to assume or be confident [the PUP] would do anything” until the party had received similar staffing resources to the Greens and had been able to fully consider legislation.

Palmer, whose Queensland Nickel company owes $6.2m in unpaid carbon tax and who has previously said the carbon tax repeal should be made retrospective to waive existing liabilities, said he did not think Abbott’s veiled threats of a double dissolution were credible “because he’d lose”.

As well as the carbon and mining tax repeals, the new Senate would likely be asked to approve any change to the legislation requiring that Qantas be 51% Australian owned.

Abbott said at the weekend it was “not unreasonable” for Qantas to want a level playing field, suggesting he may support lifting foreign ownership restrictions.

But Labor and the Greens have ruled out supporting a change in the legislation, and Palmer said his party had not yet considered the issue.

Complicating the double dissolution threat is the fact that the West Australian representation in the new Senate remains in dispute, with the high court set to consider arguments in the new year.

The court determined on Thursday it would listen to arguments being put by Labor and the PUP, and supported by the Liberal party, that a fresh Senate election in WA is not necessary and that there are legal grounds to declare a result in the poll.

In separate petitions, Labor and the PUP argue that given what is known about the 1,370 missing Senate ballot papers in the state and other mistakes they list in the counting and recounting process, the court should declare the original winners of the fifth and sixth Senate positions – PUP’s Zhenya Wang and Labor’s Louise Pratt – are elected without another half-Senate election being held.

Initial counting gave the fifth and sixth positions to Wang and Pratt, but after a recount the seats were awarded to the Sports party's Wayne Dropulich and the Greens' Scott Ludlam.

But the Australian Electoral Commission has petitioned the court that a new West Australia- only Senate poll is needed, and if it were held it could change the Senate balance of power.

The Coalition had claimed it would have a “trigger” to go back to the voters if the current Senate refused to pass the policies, which were central to the Coalition’s election campaign.

But the parliamentary year ended without a Senate vote on the carbon tax repeal – making it very difficult to achieve the necessary preconditions for a double dissolution poll until the new Senate is due to sit anyway.

The treasurer, Joe Hockey, will release the mid-year economic statement Tuesday, which is expected to show this year’s deficit has blown out from $31bn forecast during the election campaign to about $47bn.

The government will attempt to blame Labor for the blow-out.

Abbott said the Mid-year Economic and Fiscal Outlook (Myefo) would be “Labor’s last budget statement and Labor’s only truthful budget statement because what is in Myefo tomorrow will be a result of the decisions Labor has taken ... this has been the most profligate period for debt and deficit in Australia’s history, we are ruling a line under it tomorrow and the repair job starts now”.

In fact, government decisions, including the funding injection for the Reserve bank, the opposition’s blocking of savings measures, including university cuts Labor had itself proposed, and deteriorating revenue, will all contribute to the worsening deficit.

Abbott said that on current policy settings the budget would be in deficit for a decade, but said the Coalition was planning “substantial changes” which would “put us back on a path to a sustainable surplus”.

For a government to have a trigger for a double dissolution election, the Senate must twice reject a piece of legislation, with three months between the two rejections.

In the new Senate, the Coalition will need six out of eight crossbench votes. If the PUP candidate Wang is declared a senator PUP will have three senators. But Palmer has also formed a voting bloc with the Motoring Enthusiast party senator from Victoria, Ricky Muir, meaning that even without Wang PUP’s support will be needed for measures opposed by both Labor and the Greens.