The Australian-owned company told staff in a letter that they would be locked out after they voted to reject an offer that contained cuts to breaks and staffing levels worth 11% of wages and signalled their intention to strike.
EPMU organiser Cliff Gunning says the lockout notice was an opportunistic attack designed to drive down pay and conditions.
"This is a company that made profits of more than $46m last year, Amcor does very well out of its New Zealand operations but has decided to try to squeeze out even more at the expense of its Kiwi workers and then gone off the deep-end when they've been told no.
"The company's offered a pay rise but also tried to cut breaks and reduce minimum staffing at the same time which means our members would be taking an overall cut and they aren't willing to wear that. That's why they rejected the offer.
"Just last month it was announced that Amcor's CEO had his pay hiked to eight million dollars and the Aussie executive gave themselves an eleven percent pay increase but they still think they can demand cuts from their Kiwi workers and lock them out when they refuse them."
The lockout threat was lifted on Wednesday and the union spent the day negotiating with the company. Negotiations failed to make any progress, however a mediator has now been brought in and the union is hoping to make some progress next week.
The EPMU represents 66 members at Amcor's aluminium beverage can plant.