Note that the 'Royal Style and Titles Act 1973' still has his mother as 'Queen of Australia'. It hasn't been amended to indicate that Charlie might have the title of 'King of Australia'.
www.legislation.gov.au/C2004A00044/latest/text
Oz may be a de facto republic already!
There's no "or her heirs" provision in this or other legislation that allows for the entirely predictable event of the reigning monarch passing on?