A One Nation candidate in Queensland has resigned after revelations he allegedly threatened to kill an employee of his security firm in an angry diatribe in which he warned: “I know where you live.”

The alleged death threat appears in private messages sent last July from the personal Facebook account of Mark Ellis, a former police officer who was running in the state seat of Macalister.

One Nation confirmed Ellis resigned on Wednesday morning, a day after Guardian Australia put the allegations to him and the party.

Mihalis Kalaitzidis, 25, a university student who worked for Ellis’s business Gatecrash Security, took the messages and an allegedly threatening phone message from his boss to police at Upper Mount Gravatt station.

A screenshot from the Facebook page of Mark Ellis, showing a death threat against his former employee Mihalis Kalaitzidis

Kalaitzidis said an officer had told him he could “go further with it from what I’d shown him” but he decided not to risk “ruining [Ellis’s] life” by pressing a formal complaint.

But Kalaitzidis said he had decided to go public with the information upon learning Ellis was running for parliament as it showed he was “not suitable for that position”.

Guardian Australia has also obtained a picture in which the same Facebook account was tagged with what appears to be Ellis performing a Nazi salute before a large swastika mown into a backyard lawn in 2011.

It comes days after Ellis faced controversy when it emerged he was one of six police officers charged with kidnapping after taking three Indigenous boys from the centre of Brisbane and leaving them at Pinkenba on the city’s outskirts in 1994.

A magistrate dropped the 1994 deprivation of liberty charges against Ellis and the other officers after controversially finding the boys had gone willingly.

Ellis initially denied sending the threats to his former employee and questioned whether they came from one of a number of social media impersonators that had emerged in recent days.

But when told Guardian Australia had confirmed the threats came from his bona fide personal Facebook account, Ellis said he did not “remember doing that message” and that “it might be [me], it might not be, I don’t know which”.

He did not respond to a later request for comment about the swastika picture, in which the same account was tagged on Facebook in October 2011.

Guardian Australia has confirmed that police retained a copy of messages from Ellis’s account when Kalaitzidis reported the threats.

It came after Ellis, who was in the US, left what Kalaitzidis said was an alarming phone message, including an alleged threat and a warning he knew where he lived, after Kalaitzidis failed to show up for a work shift.

Kalaitzidis said he had decided to stop working for Ellis and had tried unsuccessfully to arrange a replacement.

The 30 July exchange escalates after Kalaitzidis says: “If you come to my house after leaving that voicemail. I’ll call the cops,” later urging Ellis to “just leave it be”.

Ellis’s account eventually replies: “Fuck you. I’m gonna kill you. Fuck you. You’re a cunt … I’m gonna fuck you up cunt.”

It then says: “You’re dead to me. I know where you live … Call the cops. I don’t care … See you.”

A screenshot from the Facebook page of Mark Ellis, which appears to show the One Nation candidate for Macalister doing a Nazi salute next to a large swastika mown into a backyard lawn

The deputy premier, Jackie Trad, said on Facebook she was “appalled that a man who was part of the ‘Pinkenba Six’ case has been endorsed by Pauline Hanson to run at the next state election”.

Ellis told Channel Nine of the case: “I did it, I own it, I take full responsibility for it. I wouldn’t do it again. I’ve learned a lot from it but the circumstances at that time were very different to today.”

He said he had received death threats after the TV report.

Ellis told Guardian Australia he didn’t have “anything to say about” the messages to Kalaitzidis.

“That’s an issue in our company,” he said. “It’s a work-related issue and that’s where it stays.”

Ellis then said: “Mate, I deny saying that. That might have been anyone on my Facebook profile. It could have been one of the guys that was running the office back in Australia because I was overseas.”

When pressed about his denial, Ellis said: “I’m saying I don’t know. I don’t remember it.”

Ellis declined to receive a copy of the exchange from Guardian Australia, saying: “This personal assassination really needs to stop.

“Honestly, I’ve got better things to do than to worry about still more old news that some vindictive person is going to bring up.”

One Nation’s former state secretary, Jim Savage, told Guardian Australia on Tuesday he was “not aware” of the threat allegations against Ellis.

Savage said he would pass the information on to the state campaign director, Mike Pucci, but the party executive was unable to provide immediate comment.