Over 200,000 registered voters in 25 wards throughout the nine provinces are expected to cast their votes in by-elections on Wednesday.

"The scale of the polls, which includes by-elections in a number of wards in metro councils in Johannesburg, Tshwane, Cape Town, Nelson Mandela Bay and Mangaung, makes tomorrow's elections one of the largest mobilisation of voters outside of general elections," the Electoral Commission of SA (IEC) said in a statement on Tuesday.

Out of the 25 vacancies being contested tomorrow, 12 were as a result of candidates being elected to the national and provincial legislatures after the May 7 general elections, the IEC said.

"In the previous by-elections on July 2, 2014, six of the 20 vacancies were also as a result of councillors being elected to the national and provincial legislatures."

The IEC manages an average of 130 by-elections a year.

By-elections are held when ward councillors vacate their seats for a variety of reasons including death, resignation or expulsion from the party or the council.