The Federation of Unions of South Africa (Fedusa) said it was outraged after the employment and labour minister only increased the statutory minimum wage by 3.8% to R20.76 an hour with effect from March 1.
The country's National Minimum Wage Act came into effect at the beginning of 2019, compeling all employers in the formal non-agricultural sector to pay their workers a minimum wage of R20 an hour or R3 500 a month.
The minimum wage for farm and domestic workers was pegged at 90% and 80% of those in the formal non-agriculture sector respectively.
In its statement late on Tuesday, Fedusa said the latest adjustment to the minimum wage was "totally unjustifiable and in serious conflict with the vision of ejecting millions of South Africans out of the poverty trap when the legislation was crafted during lengthy deliberations in the National Economic Development and Labour Council".
Fedusa had rejected an initial call of a 5% increase at the end of January.
"This latest move is simply undignified," it said on Tuesday, noting that the increase was below the current consumer inflation rate of 4%.
"Fedusa will call for an urgent enquiry on the rationale by the National Minimum Wage Commission. We will not allow the most vulnerable sectors of society to continuously have their disposable income further eroded whilst food, fuel and electricity prices keep increasing at an alarming rate.