No consistent wording for the warning appears in the Oaths Act 1867, Statutory Declarations Act 1959 or the JP Handbook.
Suggested warnings:
Witness: Do you understand that you may be sent to prison, fined or both, if you make a false declaration?
Declarant responds: Yes, or I do.
Witness: Do you understand that if the declaration is found to be untruthful, there are severe penalties which include imprisonment.
Declarant responds: Yes, or I do.
Witness: Do you solemnly and sincerely declare that the contents of this declaration are true and correct, to the best of your knowledge and belief?
Declarant responds: Yes, or I do.
Statutory declarations and associated references are discussed in detail in the Statutory Declarations page.