Silver Fern Farms may have to take stock after a legal challenge against changes to worker conditions.

From Monday, the tally at Silver Fern Farm's Christchurch plant will go from 525 cattle, to 600, while dropping the amount workers are paid per kill.

The Meatworkers Union is challenging the move, claming it's in breach of the collective contract.

Taylor Shaw employment lawyer Gareth Abdinor says it appears there's been a one-sided variation to the workers terms and conditions, which is a no no.

"You can't unilaterally dairy someone's terms and condition.

"You either need to get their agreement to it, or you need to go through a formal restructuring process."