Fast food chain Grill’d has found a loophole to pay thousands of workers below minimum industry rates in its latest pay deal, which a union says could slash 20 per cent from the amount weekend workers earn.

The Shop, Distributive and Allied Employees Association (SDA) vowed to challenge the national burger chain’s latest enterprise agreement, covering more than 4000 workers, for scrapping weekend penalty rates and allowances in return for a 12 per cent increase in the base hourly rate.