"(We) maintain that the toll system that is expected to be implemented on the province's main roads in June, together with the current fuel taxes, come down to double tax and calls for a discount on the current tax included in the fuel price," spokesperson Ilze Nieuwoudt said in a statement.
"Toll roads are nothing more than an inventive means of imposing tax, and the implementation of the planned toll system will overburden consumers."
Road users already paid around R2.47 per litre of petrol for various forms of tax, of which R1.67 was the fuel levy.
"This levy is supposed to subsidise the maintenance of road networks, but the money is clearly not used for this purpose," she said.
The SA National Roads Agency announced the tariffs for the 185km Gauteng Freeway Improvement Project last week.
CEO Nazir Alli said motorists who purchased the e-tag system would pay 49.5c/km. Those who didn't would pay 66c/km.
Medium-sized vehicles with the e-tag would be charged R1.49/km and heavy duty vehicles with an e-tag R2.97/km.
Motorists would get further discounts depending on when they used the highway and whether they were frequent users.
The tolling system drew widespread criticism, with concerns over its complexity and effect on the economy.
Solidarity criticised the consultation process surrounding the tolling system, arguing that the public was not given sufficient opportunity to comment.
Transport Minister Sbu Ndebele said on Monday there would be "further engagement" on the system, due to be implemented in June this year.
Solidarity further demanded that Finance Minister Pravin Gordhan deal with the issue in his budget vote.