Casual workers at GrainCorp in Port Kembla have been threatened with the sack if they don't turn up for work while others are on strike, according to a union official.
The facility is staffed by a mix of full-time workers who are members of the Australian Workers Union, and casual labour hire employees.
The 10 full-timers have been negotiating for a new enterprise agreement since December, with the old one expiring two months ago.
Those 10 full-timers will start a week of protected industrial action from Friday and AWU state secretary Tony Callinan claimed the casuals have been told they will not get any more shifts if they fail to turn up for work.
"GrainCorp management are so out of touch they think it is okay to threaten insecure, casual workers with the sack while full-time workers legally withdraw their labour," Mr Callinan said.
"It is a fundamental right of any worker in Australia to participate in protected industrial action sanctioned by the Fair Work Commission and GrainCorp shouldn't be threatening the rest of the workforce".
A spokeswoman from GrainCorp declined to directly confirm or deny the union's allegations.
"We are working to ensure all workers, including labour hire representatives, at the GrainCorp Port Kembla site are treated fairly and continue our discussions with them as part of planning for ongoing operations for the next week and beyond," the spokeswoman said.
"We will continue to bargain with the AWU in good faith, as we have done to date."
The walkout will see workers off the job until Friday, with the possibility that no grains will be loaded onto the ships due to arrive at Port Kembla.
"Based on the information from our members there are three ships scheduled to come into the port in the course of our industrial action," Mr Callinan said.
"The casual employees do not have the skills to operate the ship loader so there should be no ship-loading activities undertaken.
"We will be watching the site very carefully to ensure that, if any of those activities are undertaken, that the people doing that work are appropriately trained so their own safety and safety of others are not put it risk."
The GrainCorp spokeswoman said the company was working towards a solution to keep the Port Kembla plant operational next week.
"GrainCorp is absolutely committed to ensuring the grains supply chain continues to operate," the spokeswoman said.
"At this stage, we are planning for contingencies in the event of any disruption, in order to continue delivering our services for Australian growers and our customers."