Peter Dutton has used parliamentary privilege to accuse former Australian Border Force boss Roman Quaedvlieg of having "groomed" a woman 30 years younger than him.
But Mr Quaedvlieg demanded he withdraw the "disgusting and offensive" comment, which he said was an accusation of a criminal sexual offence.
The Home Affairs Minister verbally attacked the ex-ABF boss in parliament on Tuesday, after Labor quizzed Mr Dutton over claims he pushed for two Queensland police officers to secure jobs at the agency.
"This smear is coming from the former Australian Border Force commissioner," Mr Dutton told parliament.
"A man who was, as commissioner, sacked from his position. He was a man who had groomed a girl 30 years younger than himself.
"He is discredited and disgraced."
Mr Quaedvlieg, who was fired from his role after helping his girlfriend get a job, responded on Twitter.
"Grooming? Are you serious? That has a legislative meaning. Is that what he meant? Parliamentary privilege huh?" Mr Quaedvlieg tweeted.
In a statement he labelled it "extraordinary behaviour" from a cabinet minister.
But Prime Minister Scott Morrison is standing by Mr Dutton, saying Mr Quaedvlieg's claims have lacked "complete credibility".
"I'm disappointed that the claims that he's making now are being so palpably demonstrated to be false," Mr Morrison told ABC TV on Tuesday.
"They lack complete credibility and I'm disappointed that he has made these false claims and sought to sledge the character of Peter Dutton."
The stoush between the two former Queensland police officers originally erupted over Mr Dutton's decision to grant visas to two au pairs held in immigration detention.
Mr Dutton was also accused of lobbying Mr Quaedvlieg to secure jobs at the ABF for two other former police officers, including one who is now an adviser in his ministerial office.
"He put in for a job with Australian Border Force, there was no interference with that process," Mr Dutton said.
"He was employed ... and he was done so completely on merit."
A Senate committee is investigating Mr Dutton's decision to personally intervene to stop the deportation of an Italian nanny in 2015, after his office was contacted by a former Queensland police colleague.
The visa was granted within hours, but Mr Dutton said he intervened at short notice in many cases.
He also intervened in a similar case after AFL boss Gillon McLachlan's staff contacted his office about a French au pair.
Mr Quaedvlieg has provided evidence about what he says could be a potential third case, detailing a conversation with Mr Dutton's chief of staff about an au pair detained at Brisbane airport.
He said Mr Dutton was happy enough for him to talk to the media about a botched decision to stop people on the street in Melbourne to check their visas, and to go on TV to talk about riots in Papua New Guinea.
"But now that it doesn't suit I'm supposedly not credible? Right," Mr Quaedvlieg tweeted.
The committee will deliver its report next Wednesday, with Labor and the Greens talking to coalition MPs about the possibility of a no confidence motion in Mr Dutton.
Australian Associated Press
