Van der Westhuizen was admitted to hospital yesterday in a critical but stable condition.
The 45-year-old, who was part of the 1995 Springbok team that beat the All Blacks 15-12 in the world cup final, was diagnosed with motor neurone disease in 2011.
A post on his foundation's Facebook page said van der Westhuizen is recovering well after being placed in the intensive care unit.
van der Westhuizen played 89 times for South Africa from 1993-2003.
Radio New Zealand