A South African supreme court judge has ruled that the leader of a small, white extremist organisation is guilty of treason after he plotted to violently overthrow the country's African National Congress-led government in the 1990s.

Judge Eben Jordaan said in his ruling on Thursday that Mike du Toit, a former teacher at a segregated apartheid-era university, was the "main role player'" in a thwarted "war plan" to stage bomb attacks and kill former President Nelson Mandela.

He said Du Toit recruited supporters from among hardline white Afrikaners for his far-right "Boeremag,'' or Boer Force.

The group hoped to overthrow the ANC and expel South Africa's black majority, most of them to Zimbabwe. South
Africa's ethnic Indian population would be sent back to India by boat, the court was told. 

The group considered using the huge office complex that housed Armscor, the apartheid state's weapons company, as a breeding farm to create a new white nation.

Group leader Tom Vorster said the Armscor headquarters would be an ideal place to "keep a bunch of women, isolate them and inseminate them with Boeremag members to start a new nation".

Another 21 members were charged with treason in a trial that has dragged on over nine years. Sentencing is expected next month.

They all face life imprisonment. South Africa has no death penalty.