Around 40 striking workers picketed Steel and Tube in Christchurch, saying they will continue to strike until labour negotiations progress.
Union representatives said the action was in response to Steel and Tube removing provisions for days and hours of work, and paying workers a flat rate for all hours worked.
Plans to strike were announced after negotiations between the Engineering, Printing and Manufacturing Union (EPMU), FIRST Union, and Steel and Tube management reached an impasse last week.
EPMU organiser Steve Westoby said that "targeted strike action will continue until Steel and Tube drops its unrealistic demands and negotiates with its workers constructively".
"Steel and Tube want to be able to make workers answer their beck and call, and ignore their basic right to negotiate decent terms and conditions around days and hours of work," Westoby said. "That's simply unfair."
A spokeswoman for Steel and Tube said the company would not comment on the specifics of negotiations, but said it would "like to get a resolution as soon as possible".
She said the company was seeking "greater flexibility for our customers" and would continue to negotiate with unions until they had a resolution.
Steel and Tube has four sites in Christchurch. Workers picketed the Blenheim Rd site until 2pm yesterday.
Strikes are planned for Auckland branches of Steel and Tube next week.
Would you like to see the Heathcote River cleaned up?Related story: Alasdair Cassels to beautify Heathcote River