The union has taken out a full-page ad on Wednesday outlining its case against the company in a Fairfax newspaper.

It coincides with the a global day of action against Rio Tinto led by industriALL global union.

Titled “Rio Tinto must stop precarious work”, CFMEU northern district president Peter Jordan said Hunter miners and their families have been left ­vulnerable by the company’s work practices.

“This is affecting our economy and our people working in the Hunter Valley coalfields, where 35 per cent of Rio Tinto’s workforce is now casual contractors earning up to $40,000 less than permanent employees,” Mr Jordan wrote.

Mr Jordan said the print advertisement was designed to build on the union’s public awareness campaign. The union had previously parked billboards outside Rio Tinto mine sites to highlight the same concerns.

“This year we have decided to take it directly to the wider public,” he said.

A Rio Tinto spokesman said the company was disappointed by the unfounded allegations.

“Our number one priority is the safety and security of its workers across all of our businesses,” Mr Jordan said. “We will continue to offer a full range of employment arrangements and union agreements across our sites.

“We are committed to transparency, meaningful consultation, respect for the rule of law and the rights of communities in which we operate.”

He said the company was committed to providing employment and career development opportunities to local communities.

However, the company claims statistics from Hunter Valley mines show that while contract workers represent 35 to 40 per cent of the workforce, they represented 66 per cent of lost time injuries last year.

“The company is not transparent about the use of precarious workers and not disclose how many people work on sites it directly manages; it reports only direct employees,” Mr Jordan wrote.