Basilectal varieties of Australian Aboriginal English (AAE), which are heavily influenced by the indigenous substrate, may have a very restricted set of vowels compared to Standard Australian English (SAE). A comparison of the vowels of a small group of acrolectal AAE speakers with those of the standard accent suggests that even in varieties with the same set of phonemes as SAE, speakers are using a somewhat smaller phonetic vowel space. The lower boundaries of the AAE and indigenous language spaces are very similar and, whereas the SAE vowel space represents an expansion in all directions compared with the indigenous space, the AAE space represents an expansion in an 'upward' (lower F1) direction only. Within their respective spaces, the relative positions of the monophthongs are quite similar in SAE and AAE. Diphthong trajectories are also similar, except that some have shorter trajectories (more centralised second targets) in AAE. Most of the differences there are can be viewed as more conservative features in the AAE accent.
Bibliographic reference. Butcher, Andrew / Anderson, Victoria (2008): "The vowels of Australian Aboriginal English", In INTERSPEECH-2008, 347-350.