At a press conference on Tuesday, Warren Pickering admitted to taking speed, ecstasy and marijuana before joining the Australian Defence Force at the age of 28.
The 43-year-old former combat engineer also admitted to twice losing his licence for what he described as "low level" drinking offences at age 18 and 20.
"The mistakes I made in my 20s hurt people," he told reporters in Melbourne.
"They hurt me. They hurt people I loved and I will live with that forever."
His admissions came after the Australian newspaper published a series of allegations raised by Mr Pickering's former partner of almost 20 years, Jo Scott.
He said the days since the allegations - many of which he denied - were brought to light had been challenging, but he felt supported by his party.
"The party has been entirely supportive every step of the way," he said, confirming that extended to Senator Hanson.
Mr Pickering said he disclosed concerns about his former partner when applying to the party but some allegations had only surfaced since.
"A lot of these details came out later when it was identified that (Ms Scott) was posting things online publicly," he said.
Asked how much he had disclosed directly to Senator Hanson, Mr Pickering said the One Nation leader had a full plate.
"I make sure the right people at the time are aware of everything they need to be aware of," he said.
Among the allegations raised by Ms Scott was that the pair used drugs, including methamphetamines, in the home they shared with their daughter and her three sons from a previous marriage.
Flanked by his supportive stepson and daughter on Tuesday, Mr Pickering denied ever using methamphetamines, or ice, including in the same home as their children, or ever being a drug addict.
"(The allegations) are extremely hurtful and I just hope the public can see through them," Mr Pickering said.
He said he had engaged lawyers to advise on a potential defamation claim but did not state who any suit would target.
Mr Pickering said he hoped to provide an example to people looking to turn their lives around, claiming his challenges would be familiar to many.
"I'm proud of the man I am today," he said.
His daughter Jhett said the media scrutiny had been difficult for the family.
"My father has always been there for us ... at the end of the day, we always came first," she told reporters.
Senator Hanson previously said she was comfortable with Mr Pickering leading the Victorian branch of the party, despite the claims.
"These are allegations that have been put to Warren Pickering and he's denied those allegations," she said.
Mr Pickering, who is standing for the lower house seat of Pakenham in the November state election, previously ran in the 2022 state election and for the federal Senate in 2025 without success.
He had worked in construction and served in the army as a combat engineer and also worked in Queensland's mining industry before returning to Victoria.
Wednesday's media conference was closely guarded, with reporters from the ABC and the Guardian asked to leave before it began.
It continues a trend of the minor party turning select media outlets away.
Alliance for Journalists' Freedom executive director Peter Greste said accessibility was a key part of Australian democracy.
"Any politician deciding who gets to scrutinise them is practising censorship, and in a democracy, that's something we should never accept," he said in a statement.
Nationally, primary support for One Nation stood at 30 per cent, ahead of Labor on 29 per cent, according to a Newspoll published in late August.
But the populist party is not polling as well in Victoria.
The state coalition was the frontrunner with 31 per cent of the primary vote, according to a separate Newspoll, ahead of the incumbent Labor on 28 per cent and One Nation on 19 per cent.
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