Sistema Plastic's HQ in South Auckland. Photo / Supplied

E tū has once again alleged poor conditions at the factory, saying workers were fed up with "compulsory 60-hour weeks for minimum rates of pay".

It said staff had revealed on social media photos of blisters and calluses from their work.

E tū said that 300 unionised and non-union workers signed a petition wanting better pay.

That showed "how aggrieved people are feeling," E tū advocate Neville Donaldson said.

"Anger over their working conditions extends across the entire factory floor. When non-union workers join their unionised colleagues to protest about lack of respect, you know there's a problem," he said.

"In this case, everyone wanted to express their frustration over not being valued for their contribution to Sistema's success – something Sistema's management has refused to take on board," Donaldson said.

Chief executive Drew Muirhead said in a statement:

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