The Woolworths Select branded items, while being the cheapest, are often of a higher quality than a lot of name brand competitors.
While it is true that reputable brands sometimes manufacture the in-house brand's product, it is not (as ACA would have you believe) a case of Cadbury's quality chocolate in cost saving packaging. "Reverse tendering" occurs, where the head office "buyer" will set an objective cost and manufacturers will compete in a reverse auction towards it.
Cadbury and Nestle (and anyone who can keep up) will not simply provide their standard premium product, but look to cut as many costs as possible. What you will get is a final product that will probably be made from different ingredients, probably be manufactured with a different recipe and probably manufactured using different machinery/methods.
What you are left with is a product that barely resembles its manufacturer's name brand product, and as long as it passes unit testing/quality control, Woolies or Coles don't really care. The almighty dollar.