Speaker numbers were measured differently across the censuses and various other sources listed in AUSTLANG. You are encouraged to refer to the sources.
Speaker numbers for ‘NILS 2004’ and ‘2005 estimate’ come from 'Table F.3: Numbers of speakers of Australian Indigenous languages (various surveys)' in 'Appendix F NILS endangerment and absolute number results' in McConvell, Marmion and McNicol 2005, pages 198-230 (PDF, 2.5MB).
Steele, Jeremy Macdonald. 2005. The aboriginal language of Sydney: a partial reconstruction of the indigenous language of Sydney based on the notebooks of William Dawes of 1790-91, informed by other records of the Sydney and surrounding languages to c.1905, Macquarie University: MA.