The Public Servants Association and the Food and Allied Workers Union will join Cosatu's nationwide protest against the e-tolling system and labour brokers, the unions said on Tuesday.
"E-tolling... should be condemned as it represents yet another tax on all the citizens of this country," PSA deputy general manager Manie de Clercq said in a letter to the Congress of SA Trade Unions.
"The rationale for [the] e-tolling system is unclear in that all motorists are paying a fuel levy which can be efficiently utilised in upgrading and maintaining our public roads."
Fawu said it would join the protest to highlight the barbaric practice of labour brokers.
"This labour broker practice has seen in some workplaces up to 70 percent of the workforce being workers supplied by fat-cats called labour brokers and earning as little as 30 percent of what employees on the payroll of manufacturers earn and with huge fees paid to these fat-cats," said general secretary Katishi Masemola.
He said workers were doing permanent work and reporting for duty daily but earned far less than their fellow workers on the payroll and were working without benefits in many cases.
De Clercq said trade unions were "important vehicles" in improving the socio-economic status of their members through opposition to labour brokers.
"A united voice should speak out against labour broking, which violates workers' right to dignity."
Cosatu's protest is scheduled to take place on Wednesday.
Cosatu's Gauteng secretary Dumisani Dakile said on Monday that workers could not be dismissed for taking part in the protest.
"The strike is protected... and we have complied with the legal requirements," he said.
"We are even calling on employers to join the march and not threaten workers."
Cosatu expected at least 100,000 people to take part in 32 marches across the country. The major event would take place in Johannesburg's central business district.
Tolling of 185km of the N1, N3, N12 and R21 around Johannesburg and Tshwane is expected to start on April 30. Motorcycles with e-tags will pay 20 cents a kilometre and those without 38 cents. Light motor vehicles will pay 30 cents and 58 cents respectively, and non-articulated trucks 75 cents and R1.45.
Cosatu expected its provincial bodies, essential services workers, Eskom workers, teachers, pupils, and other unions to join the strike.