Australian languages show close similarities in their systems of consonant and vowel phonemes, in their canonical phonotactic structure and in stress placement; these are outlined in §12.1. Certain types of diachronic change tend to recur in different parts of the continent. The general picture is of overlapping diffusion areas for various features; and of changes engendered by the internal dynamics of a system taking place independently in scattered locations across Australia. Many of the changes are cyclic: a certain contrast may be innovated, then lost, then – a long time later – acquired once more, each development being a consequence of a rolling pattern of areal change.

There are two significant phonetic characteristics of Australian languages. Butcher (forthcoming) has pointed out that the lowering of the velum for nasal consonants tends to be delayed as long as possible. This has two main consequences. Firstly, there is relatively little phonetic nasalisation of a vowel preceding a nasal consonant in an Australian language. Among the various types of diachronic change, the development of nasal vowel phonemes scarcely features (these are reported just for one language, Ba6, Mpakwithi – Crowley 1981: 154–5). Secondly, in some languages the lowering of the velum is delayed past the commencement of the consonant articulation (this is especially so when there is no nasal consonant earlier in the word) resulting in a prestopped nasal; that is, the stopping commences prior to the nasal articulation.