In a decision published on Tuesday morning, the Fair Work Commission ruled that the Coles EBA, which covers some 77,000 employees, failed what is known as the better off overall test (BOOT).
For a workplace agreement to be legal, all employees must be made better off when an agreement is compared to the award.
Duncan Hart, a student who works part-time at a Coles supermarket in Brisbane, took action in the Fair Work Commission last year.
He claimed the enterprise bargaining agreement between Coles and his union, the Shop Distributive and Allied Employees Association (SDA), left thousands of workers worse-off than they would be under the award, and was therefore invalid.
The commission gave Coles 10 days to remedy the failings in the EBA before it quashed its original approval of the workplace agreement.
The decision could force Coles to renegotiate wages and conditions for its workforce of 77,000 employees.
The Coles agreement in question, which was approved by the Fair Work Commission in July last year, mandated a higher hourly base rate for supermarket workers, but cut penalty rates for weekends and nights.
Mr Hart argued that this left a substantial proportion of the Coles workforce worse-off than if they were paid under the existing award.
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