Unions will challenge the workplace umpire’s landmark gender pay decision over concerns it perversely cuts social workers’ pay by hundreds of dollars a week, clashing with employers who say the decision aims to simplify the complex and confusing system.

The Australian Services Union warned last week’s historic Fair Work Commission decision to address the gender undervaluation of tens of thousands of community and disability support workers’ work would inadvertently send their pay backwards by up to 20 per cent.