More than 850 NZ Steel workers are striking for 24 hours at Glenbrook and Taharoa in the Waikato, following a break down in talks over their collective agreement.
The Engineering, Printing and Manufacturing Union (EPMU) said the sticking point with NZ Steel was rates of pay and the use of contractors on site.
"If steel workers employed on site do not get a say on how contractors are used and paid, then the workers will find their own pay and conditions undermined,'' said EPMU senior organiser Stuart Hurst.
For members to strike was a serious step, he said.
"The site has enjoyed industrial stability for 20 years, and there have been no problems or difficulties during maintenance shuts or other times when contractors are needed.''