A Sistema worker's hands were allegedly blistered from handling product on the manufacturing line, the union says.

Staff are "fed up" with being paid minimum wage while working 60 hour weeks for New Zealand plastic container manufacturer Sistema, a union says.

Sistema, which was bought for $660 million 18 months ago by a US Fortune 500 company, produces containers that have become a staple in New Zealand kitchens and workplace tearooms.

The union representing workers at Sistema says the company is not only failing its staff on pay rates, but health and safety as well.

BEVAN READ/STUFF Sistema Plastics managing director Brendan Lindsay sold the company for $660 million.

But the company has hotly denied the allegations - which includes a social media post of a worker who apparently blistered his fingers working at the company's Mangere factory.

READ MORE:

* Sistema sold to Sharpie owner US Newell Brands for $660 million

* Sistema founder spends his fortune on those who can't help themselves

Et tū union has been in talks with the company to try to secure better pay and conditions for its mainly migrant workers. Of the 500 production staff that work at its purpose built Mangere plant, about 200 are union members.

SUPPLIED The 55,000 square metre Sistema plant in Mangere, south Auckland, was opened by former Prime Minister John Key.

E tū advocate Neville Donaldson said more than 80 per cent of its workers were paid minimum wage of $16.50 an hour.

Staff were required to work five 12-hour shifts a week, with some of the shifts being graveyard, he said.

Workers were also required to pick up additional work and "overtime" rates were just $2 more per hour, he said.

The union was calling on Sistema to introduce a living wage of $20.50 an hour over the next three years, but the company would not agree to that condition, he said.

That is despite New Zealand's minimum wage increasing to $20 per hour by 2020.

"They're just refusing to offer anything other than what the statutory requirements are in law."

In a written statement Sistema chief executive Drew Muirhead said the union's claims were incorrect.

"We strongly refute the outrageous claims made by the union that are full of half-truths and inaccuracies," Muirhead said.

"We have been in negotiation with them for several months and while their tactic may be to negotiate via the media we will not be participating."

More than 300 Sistema workers had signed a petition seeking better pay, Donaldson said.

Both union and non-union workers signed the petition, he said.

"Anger over their working conditions extends across the entire factory floor," Donaldson said.

"When non-union workers join their unionised colleagues to protest about lack of respect, you know there's a problem."

Workers were fed up with compulsory 60-hour weeks for minimum wage, and "terrible working conditions", Donaldson said.

At the weekend the union posted to Facebook a photo of blistered hands - allegedly a worker's, injured from the work they do at Sistema.

The injury was caused by hot moulds coming off the production line, Donaldson said. Such injuries were common, he said.

High staff turnover at Sistema had resulted in low levels of staff education in health and safety, he said.

One delegate told E tū Sistema was a "sweatshop", the union said.

Donaldson said Sistema had refused to address workers' concerns.

The union had held concerns about conditions at Sistema since before it was sold but had not been able to take action due to low collective agreement numbers, Donaldson said.

Founder Brendan Lindsay, who held a 90 per cent stake in the company before it sold to Newell Brands, said at the time of sale all staff would continue with their existing employment conditions.

"It's business as usual, nothing changes," Lindsay said at the time.

Lindsay stepped down as managing director of the company he started 34 years ago from a garage in Cambridge.

Dundas Street employment lawyer Susan Hornsby-Geluk said the Minimum Wage Act dictated that the maximum number of hours employees could work per week was 40, unless agreed otherwise.

If workers had agreed to work more than 40 hours it raised a question over whether that was safe, she said.

"If people are being required to work 60 hours a week I think there would be potential health and safety risks arising out of their fatigue, tiredness and overall stress," Hornsby-Geluk said.