This is a brief video by the Therapeutic Goods Administration on how they assess the therapeutic products they will approve for use in Australia.
Every therapeutic product (including the "natural" ones) is meant to cause changes in the body - which means that they all have risks as well as benefits.
When the TGA approves a product, that does not mean the product is "harmless" or is safe for non-approved uses. It means that the benefits outweigh the risks for the particular purpose the product's been approved for.
The TGA keeps a register of every product it has approved for use in Australia, you can go to their website and search medications you are taking (both prescription and non-prescription) and find information about the product, including any recalls or other press releases, and often consumer safety information.
The internet's made the world a much smaller place, but beware of where you buy any medicines. If you buy from outside Australia, you may get a product the TGA hasn't approved, or may not be made to the specifications the TGA's approved, and it may even be an illegal import. (So it's possible to buy a medication that can both make you sick and make you party to an illegal activity.)
Oh, and if you do have a problem with a regulated product, make sure you (or your doctor on your behalf) report it to the TGA - that's vital information the TGA needs to keep doing its job properly.