Workers at the Tiwai Point aluminium smelter will begin industrial action on Monday after bargaining talks with Rio Tinto broke down.
Around 185 E tū union members are set to take action on May 4, 6, 8, and 10 following months of unsuccessful negotiations that began in 2024.
E tū Director Mat Danaher said the dispute centres on workers' basic rights to organise collectively.
"We believe this failure to agree is a deliberate anti-union tactic," he said. "They do not want to have a collective employment agreement in place. It feels like they would much prefer workers to continue to be individuals who they can push around and kind of decide what they want to do and basically deny the workers the right to a voice."
Danaher pointed to Rio Tinto's track record of anti-union behaviour, noting that Australian workers have also taken industrial action against the global mining giant.
"This is a hugely profitable global company," Danaher said. "It reported underlying EBITDA (earnings before interest, taxes, depreciation, and amortisation) of $US25.4 billion ($NZ43bn) and profit after tax of $US10bn ($NZ17bn) for 2025, and announced ordinary dividends of $US6.5bn (NZ$11bn). The workers who keep Tiwai running deserve a fair share of that success."
Production worker and delegate Dee said workers' demands are reasonable.
"We're not being unreasonable. What we want is decent work. We want an agreement that recognises the job we do, the conditions we work under, and the contribution we make."
Rio Tinto maintains it offers competitive terms and conditions that position the company well in both Southland and the wider New Zealand market.
A company spokesperson said they would continue to engage with the union and staff in good faith, confirming mediation will resume on May 20.
"In a tough economic environment for many businesses, we're proud to continue to offer our team members and those looking to join our team market leading benefits and we will continue to do so as part of Tiwai continuing to be a great place to work," the spokesperson said.
The company said its priorities remain "safety and environmental stewardship, and stability of our operations and of our local communities".
Danaher described industrial action as a last resort, but said Rio Tinto needed to understand that Tiwai Point workers could not be taken for granted.