Lol very naive, legislation creates the basic outline of an area that the government may move into. Anything reasonably in that area of responsibility if not specifically denied in the legislation is completely up to the department and minister, ceo / executive and minister *until the government says otherwise*.
The legislation doesn't mention the technical setup, the topography the subnets, how many switches, which brand of switches and servers, anti-virus policies and spam policies or anything else to do with the technical runnings of the network.
That's because it will be up to the department to come up with those policies and implement all those in the normal course of its operation.
Are you saying the ISP will offer no inline anti-spam, anti-virus measures because they're not specifically outlined in the legislation?
The department can do anything it likes regarding the day to day operations of its area of responsibility, and I can well assure you having been employed in three federal departments, brother, sister in-law, ex-wife, father in-law, mother in-law and various friends currently employed by the Commonwealth Government, policy decisions far bigger than implementing anti-childporn filters on public networks are made by public servants in The Nations Capital every single day.
There will be a policy decision made to implement filtering on the public network, that's how these things work, they will justify it in front of the Senate the same way they justify most of the things that government departments get away with that they have no authority for, if the Senate doesn't like it they will ask the government to implement legislation to stop it.
That's way the Australian Federal Government operates.