My boss is the Director of HR where I work, we deal with this almost daily (it is a big workplace). I'm not sure if it's the SA Employment Standards, or the National Employment Standards (I think it's national) - Maternity leave should commence 6 weeks prior to the expected due date, but can be up to 2 weeks prior if a doctor or midwife signs a medical certificate saying that the mother is fit to continue working beyond 6 weeks before EDD.

This next part I'm not sure if it's an Employment Standard rule or a specific rule to our place of work, if the employee needs to take leave before 6 weeks prior to EDD then it should be taken as other leave (eg: sick, annual, long service, leave without pay, etc) and a medical certificate will need to be provided.

However, if a baby is born early, maternity leave will commence from the child's date of birth.