Two opposition parties are separately challenging the constitutionality of a section of the Electoral Act which renders thousands of South Africans living abroad unable to vote in the general elections.

And according to constitutional expert professor Pierre de Vos, they "may well have a reasonably strong case".

The DA has already filed papers in the Cape High court and will for now only seek to ensure that South Africans who are temporarily living abroad may cast their vote.

The Freedom Front Plus (FF+) is preparing to lobby the Constitutional Court to demand that the constitutional rights of all South African citizens, regardless of the length of time they have been abroad, are upheld.

It is understood that both parties believe that the restrictive measures excluding many South Africans from voting are unconstitutional.

During the first democratic elections all South Africans living abroad were entitled to vote, but legislation was subsequently amended.

Currently, section 33 of the Electoral Act dictates that only South Africans who are "temporarily" absent from South Africa for the purposes of holiday, business trip, attendance of a tertiary institution or participation in an international sports event may vote, and only if that person informs the Independent Electoral Commission (IEC) within 15 days after the proclamation of the election date.

But while the Electoral Act disqualifies large numbers of South Africans from casting their vote by reason of their geographical location, the Constitution states that "every adult citizen" has the right to "vote in elections for any legislative body established in terms of the Constitution".

Foreign affairs spokesperson Ronnie Mamoepa says it is impossible to determine how many South Africans live abroad, but it is arguable that if all South Africans outside the country were able to exercise their voting rights, these voters could make a measurable impact on the election results.

According to De Vos, legal precedent suggests that a challenge to the Electoral Act could be successful.

"The Constitutional Court has said that the right to vote is fundamental to democracy, and that this required proper arrangements to be made for its effective exercise," said De Vos.

This meant the legislature and the executive should provide the legal framework, infrastructure and resources for free and fair elections.

"South African citizens who live abroad can argue that their constitutional right to vote is being infringed because the Electoral Act in effect denies them a right to vote.

"If such an application is brought, the government will have to provide solid reasons why these citizens are being denied their right to vote as they will have to show that the limitation of this right is justified in terms of the limitation clause in the Bill of Rights," said De Vos.

ID leader Patricia de Lille said in 2003 that government's excuse that it lacks the resources to accommodate overseas citizens was "unacceptable", as the same resources that made provision for government officials abroad should be used to facilitate the voting procedure for non-government employees.

She said prisoners awaiting trial were allowed to vote, and other South African citizens who are out of the country should also have this privilege.

FF+ leader Pieter Mulder said that he believed that restrictions placed on expat voters were a "purposeful" way to exclude their votes.