Acts can repeal existing rules of law. For example, the Copyright Act 1968 (Cth) repealed earlier Copyright Acts.
Acts can also repeal common law rules. For example, the Crimes Act 1958 (Vic) abolished the common law rule that husbands could not be found guilty of the rape of their wives.
In the present case, there is no suggestion that repealing an existing law is needed to implement the government's new policy.