The Greens and Labour have slammed a plan by meat company Affco to lock out 762 workers at five plants across New Zealand from Wednesday.

Affco wants workers to accept major changes to a collective employment agreement, giving the company total employment flexibility, Meat Workers Union secretary Dave Eastlake said on Saturday.

Greens industrial relations spokeswoman Denise Roche said owner Talley's Group could not be trusted to negotiate a collective employment contract in good faith.

"The workers shouldn't have to accept the casualisation of their jobs for Talley's to agree to the collective employment contracts," she said.

Labour's Darien Fenton said employers have become emboldened by the return of the National-led government.

"There appears to be a growing determination among some employers to casualise and de-unionise their workplaces, as we are seeing at the Ports of Auckland," Ms Fenton said.

Customers have moved business from Ports of Auckland due to ongoing industrial action by 300 workers.

The Talley family was estimated to be worth $300m in the 2011 National Business Review rich list. It built a fishing business up, and in 2010 took over Affco, which was then the fourth-largest meat processor in New Zealand.

The meat industry has struggled while the dairy industry has boomed and attempts to restructure it by merging companies have failed in the past.