The NSW anti-slavery commissioner has sparked an extraordinary dispute with NSW Nationals leader Gurmesh Singh after alleging the Coffs Harbour politician has “been on notice for a long time” about the exploitation of migrant workers by unscrupulous labour hire firms operating in his electorate.

Singh categorically rejected commissioner James Cockayne’s claims, describing them as a “political statement”. He called on the commissioner to apologise, saying no evidence had been furnished to support his “unsubstantiated slur”.

Singh, whose family built a blueberry and macadamia farming empire on the state’s north coast over successive generations, spent six years as the chairman and founding director of Oz Group, a Coffs Harbour co-operative with 150 members, until he entered NSW Parliament in 2019. He also served as vice president of the Australian Blueberry Growers Association.

The NSW Anti-Slavery Commissioner’s submission to a parliamentary inquiry, made public on Monday, alleged a system of exploitation overseen by labour hire companies was exposing migrant workers to modern slavery risks and “actual modern slavery harms”.

Berry farmers provided produce to major regional suppliers such as Oz Group, Driscoll and Costa, who sold it to major supermarkets and distributors, Cockayne said. Confidential tip-offs made it clear “some” farmers were intimately aware of the operation of labour hire syndicates.

“There are real questions to answer about who in those buying groups knew what, and when, regarding the treatment of these workers,” Cockayne said, explaining there were “around four to six” labour hire syndicates with connections to berry farmers across the region.

“This is an allegation that goes back many, many years, and actors such as Mr Singh have been on notice for a long time. There’s also a question to answer about why this system of exploitation has been allowed to persist in the region.”

One of the answers is “intimidation and harassment” of whistleblowers, Cockayne said, while another is the “deliberate cultivation of political influence within local and other political bodies across the region and beyond”.

Singh, who claimed the Nationals’ leadership after the abrupt resignation of Dugald Saunders in November, commanded “unparalleled influence” in Coffs Harbour and the Nambucca Valley, Cockayne alleged.

In a statement, Singh said: “I categorically reject the political statement made by a senior NSW public servant this morning. No evidence was provided to back up his unsubstantiated slur.

“Throughout my time in the industry, and in the eight years since, I have worked tirelessly to weed out bad actors, and support strong regulation of labour hire to better protect employees and farmers.”

Singh said Cockayne had not raised these issues with him despite having his phone number.

“I welcome his phone call and apology,” he said.

Without citing specific evidence, Cockayne’s investigation suggested “interlocking patterns of conduct by labour hire companies and syndicates, berry farm operators and owners, and buying entities, that have produced systemic labour exploitation in the Coffs Harbour and Nambucca Valley region”.

Confidential evidence detailing how labour hire syndicates were operating in a way that amounted to modern slavery had been provided to the parliamentary committee and law enforcement agencies, Cockayne said.

Cockayne’s submission said the system of labour supply and the exploitation risks were a badly kept secret, and unlikely to be unique to the region.

“We all know there is a system of labour exploitation by dodgy labour hire companies that some berry producers rely on. But we are too scared to name it. And government agencies just can’t see what is right in front of them,” an informant quoted in the commission’s submission said.

Different law enforcement agencies were each aware of “part of the problem” and no one had put the “pieces together”, the submission stated. But the investigation contended “it was possible” syndicates had cultivated influence within agencies to avoid effective inspection and regulation.

In a joint statement on Tuesday afternoon, Police Minister Yasmin Catley and Work Health and Safety Minister Sophie Cotsis said the government was “monitoring” allegations of illegal labour practices on berry farms.