Mr Nkanunu was praised as a force for good in the game and a builder of bridges who had come to the sport’s rescue at a time of extreme controversy, said current president, Mr Mark Alexander.
“Silas had many great qualities and skills as an attorney but perhaps the greatest tribute you can pay is that he was a simply a good man,” said Mr Alexander.
“It might sound like faint praise, but he had none of the ego, malice or aggression that sometimes characterises our world and he was an immensely calming and consensual influence at a very difficult time for rugby.
“In some ways you might say he was rugby’s Mandela in that he was a force for good and was able to completely steady the ship in his time in office.”
The human rights lawyer succeeded Mr Louis Luyt as Union president in September 1998 to become its first black African leader.
The Port Elizabeth-based attorney succeeded Mr Luyt who had been ousted when rugby provinces rebelled against his leadership style following confrontations with government and the National Sports Council (NSC).
Mr Nkanunu had been a member of the Union’s executive since rugby unity in 1992 and had served as senior vice-president, before he resigned in protest at Mr Luyt's tactics early in 1998. He was elected unopposed for the position of president after Mr Luyt’s removal.
He held the role until the end of 2003 and a disappointing Rugby World Cup year when he stood down in the face of opposition from Mr Brian van Rooyen who succeeded him as president.
Mr Nkanunu, who had been chairman of the IRB’s game’s regulation committee, had been elected as the global body’s vice-chairman only the month before.
At the time of his election, IRB Chairman Syd Millar, said: “Mr Nkanunu is one of the most distinguished rugby administrators in the world. Having done so much in helping unite South African rugby he has gained the respect of the rugby world and therefore there can be no more fitting appointment.”
Mr Nkanunu, who ran his own law practice in Port Elizabeth, was also Chairman of the Law Society of South Africa.
“No one who met Silas will be able to do anything other than remember him with great affection,” added Mr Alexander.
“Players talk about being able to leave the jersey in a better position than when you first received it. Silas was certainly able to say that.”