Who decides on the final form of the ballot text?

Australia
See generally: Graeme Orr ‘The Conduct of Referenda and Plebiscites in Australia: a Legal Perspective’ (2000) 11 Public Law Review 117
And also: Referendum (Machinery Provisions) Act 1984 Schedule 1 Form B
The referendum ‘question’ is effectively set by the title of the proposed law/bill to be approved; the Australian Electoral Commission then formats the ballot in accordance with Referendum (Machinery Provisions) Act 1984 Schedule 1 Form B.