Prime Minister Anthony Albanese has criticised Liberal MP Stuart Robert for failing to attend parliament, despite continuing to be paid as a shadow minister.
Mr Robert, the federal member for the Gold Coast seat of Fadden, announced this month that he would step down from politics to focus on his family.
But he is yet to formally resign, which Mr Albanese said was "completely unacceptable".
"If you're a member of parliament, your basic duty — unless there's a good reason why you're not in parliament with other duties or ill health — you have to turn up," Mr Albanese said.
Mr Robert has missed three parliamentary sitting days since his announcement.
"He continues to be paid as the representative of the people of Fadden, and they deserve better," Mr Albanese said.
Mr Albanese made the comments earlier on Wednesday, before travelling to Mr Robert's Gold Coast seat of Fadden to tour a space technology company.
"He said that he's resigning but he hasn't resigned. So, no date can be set for the by-election, but he's not turning up to parliament," he said.
"That is not business as usual."
On Tuesday, opposition leader Peter Dutton said there was nothing unusual about Mr Robert's announcement without formally following through with his resignation.
"I just don't think that's unusual at all," Mr Dutton said.
"There's a preselection process underway in both parties obviously to preselect candidates and there will be a decision made by the speaker to set a date for the by-election, that's a normal course of events."
A date for the by-election cannot be set until Mr Roberts submits a formal letter of resignation, according to the Australian Electoral Commission.
Mr Albanese said former prime minister Scott Morrison had continued to attend parliament amid speculation that he had "foreshadowed leaving at some stage".
He also gave credit to former Liberal MP Alan Tudge for his resignation earlier this year.
"Give Alan Tudge credit, at least when he said he was resigning, he stood up in Parliament, gave a speech, and was off a week later," Mr Albanese said.
Mr Tudge's resignation triggered a by-election in the Victorian seat of Aston, which resulted in the first opposition by-election loss in more than a century.
A by-election in Mr Robert's safely held seat would be the second by-election Mr Dutton faces this year.
The ABC has attempted to reach Mr Robert multiple times for clarification on his resignation since it was announced.
A staffer at his Labrador electoral office this week said Mr Robert was not in the building.
In response to Mr Albanese's comments on Wednesday, Mr Robert's office said he would not comment further and referred to his resignation statement from May 6.
In the statement, Mr Roberts said: "For now, I will fulfil some final duties as Member for Fadden over the next few weeks before becoming a full-time husband, father and son."
