Van der Westhuizen won the World Cup with the Springboks in 1995.
Regarded as one of the finest scrum-halves in history, he won 89 international caps between 1993 and 2003, scoring 38 tries.
He captained the Springboks for four years, including at the 1999 World Cup, before his retirement in 2003.
Van der Westhuizen was admitted to hospital in Johannesburg on Saturday, when he was said to be in a "critical condition".
