Todd Barclay has been working in his electorate office on constituent matters, National says - as Winston Peters criticises the handling of his employment as "rotten".
Barclay, 27, hasn't been at Parliament since announcing last month he would not seek re-election as the Clutha-Southland MP, after allegations he secretly recorded a staff member during an employment dispute.
Parliament is currently in recess but will return next week.
A spokeswoman for Prime Minister Bill English referred questions about Barclay to senior whip Jami-Lee Ross, who said no decisions had been made about whether Barclay would return for the final four sitting weeks before September's election.
"As the party whip I am in regular contact with [Barclay] ... he has said he is keen to continue to work as MP for Clutha-Southland until the election and I am aware that he has been working in his office as the local MP on constituent matters."
Barclay did not respond to a request for comment.
Speaking at the NZ First annual conference in Manukau, Winston Peters referenced Barclay when listing why National's claim of providing stable government was bogus.
"This government reeks of instability from the very top, all the way down to Todd Barclay," Peters said.
After his speech, he carried on the criticism, saying Barclay should not be drawing an MP salary if he wasn't prepared to show up at Parliament.
"They think there is one law for them, a different law for everybody else ... in fact, it is quite rotten, actually."
Police have re-opened an investigation into allegations Barclay secretly recorded the conversations of his former electorate staff member Glenys Dickson.
When Barclay announced on June 21 he would not seek re-election it was roughly three months to go until the election, a period in which Barclay will have the benefit of taxpayer-funded travel, a taxpayer-funded car and his $40,000 salary - a quarter of the basic MPs' pay packet of $160,000.
Those benefits and another three months pay then come to Barclay as part of the arrangement enjoyed by politicians who are leaving Parliament.
Green Party co-leader James Shaw has said Barclay should have quit Parliament immediately.