Under Australian Law, if a document was found to be legally invalid, can a third party tell the originator to remove parts of it and re-issue it with the same date as the original document, effectively back-dating it?
Certainly the 3rd party can request it. Doesn't mean you have to follow the request of a 3rd party. If you agree with the changes then just do it.
Much will depend on what is wrong with the document. If the error is serious enough, the whole contract/agreement is not valid and cannot be enforced. If the invalid section is minor, just that section may be severed from the agreement without affecting the rest of the agreement.
You may need a lawyer to review the agreement before deciding what you want to do.