Can we opt not to accept statuary declarations, pharmacy certificates and e-certificates if it is written in a company leave policy and employees have been given notice?
No, your policies cannot include this. Under the National Employment Standards (NES), the evidence (medical certificate or statutory declaration) must satisfy a reasonable person.
If you do not reasonably believe the certificate to be truthful, you can dispute its legitimacy (but this will be rare) and you should have strong evidence to suggest a medical certificate is illegitimate. Further, a deliberately false statutory declaration is an offence.