Labor has taken aim at two Liberal candidates who have failed to publish evidence they are eligible for federal parliament.
Under new rules, candidates for five federal by-elections have been asked to voluntarily fill in a checklist showing they are not disqualified under the constitution from running for parliament.
All five Labor candidates completed the checklist and allowed it to be published.
Liberal candidate for Mayo, Georgina Downer, lodged a form but asked that it not be published.
"It is disappointing that the Liberals think they're above the rules," Labor's electoral spokesman Senator Don Farrell told AAP on Thursday.
"Georgina Downer was born in the UK but she's keeping her citizenship checklist secret from the people of Mayo."
Ms Downer told reporters in May, when she was preselected, she had formally renounced her dual British citizenship in September 2017.
Senator Farrell said the Liberal National Party's candidate for Longman, Trevor Ruthenberg, was born in Papua New Guinea but "failed to produce any evidence that he is not a PNG citizen".
"The whole purpose of this process is to increase transparency and give the public confidence in these processes - the Liberals' non-compliance is at complete odds to this," he said.
The Australian newspaper reported Mr Ruthenberg had a letter from PNG's Office of Immigration and Citizenship Authority stating neither he nor his parents were a citizen of the country and there were no records indicating he had ever applied to become one.
Four of the five by-elections were triggered by MPs' dual citizenship issues, three of whom hailed from the ALP.
The government has signalled it wants the process to be compulsory for the next federal election, but does not favour changing the constitution to avoid future problems.
Senator Farrell said Labor would consider any electoral law changes on their merits.
The Australian Greens described the optional disclosures as a "band-aid solution".
Meanwhile, more than 5000 people have cast a ballot in the first two days of early voting in Mayo, Longman, Perth, Fremantle and Braddon.
Labor leader Bill Shorten was in Longman on Thursday announcing a $17 million care clinic on Bribie Island.
Mr Shorten said it was the sort of project that could be funded if the government did not provide a $17 billion handout to big business, through its corporate tax cuts.
Prime Minister Malcolm Turnbull, who was on the Gold Coast to discuss the childcare reform rollout, told reporters he would visit Longman next week.
"It's a big country ... (but) there's a very clear message for the people of Longman today: Bill Shorten is there planning to jack up their taxes, raid the retirees on Bribie Island's savings," he said.