The Council of Trade Unions says proposed changes to Accident Compensation Corporation workplace levies could leave workers worse off.
From 1 April, levies will be adjusted in line with workplace's history of accidents.
Larger businesses would be eligible for a discount or penalty of up to 50% based on their claims history over the previous three years, while smaller businesses would face a system of no claims bonuses and high claim penalties.
CTU president Helen Kelly says rewarding employers for lower claims rates provides incentives for them to cover up accidents or bully workers into not seeking treatment.
But Business New Zealand's Phil O'Reilly says it is in employers' interests to make sure their workers are properly rehabilitated after an accident.
And Federated Farmers' ACC spokesperson Donald Aubrey says a system that rewards those with good safety records is fairer and will improve workplace safety.
The Government says the change should improve workplace safety and provide a fairer system.