Mr Lew said that as the former chief executive of his back to school stationery brand Smiggle, John Cheston had encouraged a culture of drinking and gambling with staff, among other misdeeds.
"I'm appalled that this was going on for some years and I never knew about it," Mr Lew told reporters on Thursday.
Premier Investment's Just Group fired Mr Cheston in September 2024, three months after he gave his required year-long notice that he was leaving to head Lovisa.
Mr Cheston's legal representative, Rebekah Giles, said her client had declined Premier's offer to lead a separately listed Smiggle business and instead resigned to accept a CEO role for another publicly listed retail company.
"That decision appears to be what Premier is unable to move past," she said in a statement, describing Mr Lew's allegations as a "rant" that was "simply untrue".
"Mr Cheston has moved on and Premier should do the same," she said.
Mr Lew said however, that a "very serious" investigation by a law firm was ongoing and people had been interviewed.
"People were being paid to be at work, doing other things, and not being at work, and being absent from their jobs, and spending time being intoxicated and gambling with staff in the organisation," Mr Lew said.
This happened during office hours, and head office staff would return to work intoxicated, Mr Lew alleged on a media conference call as Premier announced its full-year earnings.
Mr Lew also alleged there was workplace bullying that was never reported, and that Mr Cheston had interfered with the human resources department about it.
"There are people who were dragged into this mob, obviously, over a period of time - quite a few people who are no longer with the company," he said.
Mr Lew said he did not believe anyone else had been fired over the alleged misconduct, but they had left the company.
Others who were bullied had left as well, he said.
"If you were to ask me whether those practices are going on today, I'm going to say no," he said.
Just Group publicly announced Mr Cheston's firing a year ago for what it described as serious misconduct but had not gone into specifics until Thursday.
Mr Lew said Premier had not paid Mr Cheston any further remuneration after firing him, clawing back $5.2 million.
Mr Lew said Smiggle still did not have a permanent CEO because Premier was being "very, very particular" about finding a replacement.
Smiggle has 296 stores, about half of them in Australia, and about 80 head office staff.
Mr Lew's comments largely overshadowed Premier's 2024/25 earnings results.
Premier Investments reported $831.3 million in revenue in the 52 weeks to July 26, up 1.2 per cent from a year ago.
Sleepwear chain Peter Alexander's 2024/25 sales were a record $548 million, up 7.7 per cent from a year ago, with sales around Mother's Day and Father's Day particularly strong.
Premier said its statutory profit was up 31.1 per cent to $338.2 million, which included the divestment of its clothing stores Just Jeans, Jay Jays, Portmans, Dotti and Jacqui E to Myer in January.
Its underlying profit from continuing operations - excluding the apparel brands divestment and costs associated with Peter Alexander's entry into the UK market - was down 14.9 per cent to $220.3 million.
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