The Plymouth Brethren Christian Church and Advance, a right-wing advocacy group, have declined invitations to appear at three rounds of hearings, prompting the committee to threaten a rare summons.
"This is an extraordinary step, but one (the committee) believes is necessary," Labor MP and inquiry chair Jerome Laxale said in a statement on Thursday.
The two groups campaigned vigorously during the 2025 election.
Advance raised $13 million for its effort, mostly to fend off Greens candidates, more than all other campaigners excluding political parties and candidates.
More than 75 entities also wrote to the inquiry about Plymouth Brethren congregants campaigning in about 80 suburbs across Australia, Mr Laxale said.
Rival campaigns reported the church was responsible for an unusual abundance of Liberal Party volunteers outside polling places in the lead-up to election day.
Many of the Brethren's members abstain from voting because they believe society's leaders should be ordained by God.
"It is not just in the committee's interest, but Australia's interest, to understand the involvement of both of these third parties in the 2025 federal election and their influence on the electoral process," Mr Laxale said.
The church, which has about 15,000 members in Australia, has denied mobilising its members to campaign, saying they all volunteered individually.
In a letter posted on the church's website on Wednesday, Plymouth Brethren director Lloyd Grimshaw alleged the parliamentary committee had conflicts of interest, selected witnesses with bias, and mishandled confidential information.
"This pattern of behaviour is not consistent with an impartial parliamentary inquiry. It has the hallmarks of a taxpayer-funded campaign against our church," Mr Grimshaw wrote.
The church says it has offered to attend hearings three times but could not do so on the dates they took place.
It offered to front the committee at a later date, after its chair returns from watching the World Cup in North America.
The inquiry is in the hands of the federal parliament's Joint Standing Committee on Electoral Matters, which investigates the conduct of every federal election as a matter of routine.
Failure to obey a summons can result in six months' imprisonment and a $5000 fine.
The committee is yet to set dates for future hearings.
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