The by-election in the Queensland seat of Fadden was triggered following the resignation of controversial former Liberal minister Stuart Robert.
Mr Robert served as the veterans' affairs, national disability insurance scheme and government services minister under the Turnbull and Morrison governments.
He is alleged to have been linked to a Canberra lobbying and consulting firm that helped companies secure lucrative government contracts.
A government review launched by now-Government Services Minister Bill Shorten found 19 contracts needed further investigation over a lack of appropriate record keeping and were not considered as having delivered bang for buck.
Mr Robert denied any wrongdoing and said there were no findings of clear misconduct.
Prime Minister Anthony Albanese said ALP candidate Letitia Del Fabbro, a local nurse educator, was starting at "very long odds".
But he criticised Mr Robert for not attending parliament to explain why he was resigning "under such a cloud".
"People will ask why is it that this money is having to be spent on this by-election with someone who just has made that resignation," he told Sky News on Sunday.
"We think this is unnecessary and shouldn't have happened."
Mr Albanese said Ms Del Fabbro was a strong candidate and he would launch her campaign in the next few days.
Fadden is held by the LNP with a margin of 10.6 per cent and the party has chosen Gold Coast councillor Cameron Caldwell as its candidate.
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