Abstract Phonologically contrastive vowel length has evolved — as an ‘areal feature’ — in the Yidiny language and in the contiguous Ngajan and Wari dialects of Dyirbal, in North Queensland. The origin of the length contrast in Ngajan is examined in detail, especially as it ‘compensates’ for the loss of a rhotic contrast in this dialect. It is suggested that the continuant rhotic naturally engendered lenght on a preceding vowel and that this length then became the contrastive feature, with the continuant and trill rhotics falling together as a single phoneme. Vowel length then spread both (i) within Ngajan, through the replacement of syllable-final l and y by length on the preceding vowel; and (ii) by diffusion of some of the Ngajan changes into the Wari dialect. A number of allomorphic alternations of grammatical inflections are traced back to diachronic changes, which are dated with respect to the eight reconstructed stages in the evolution of contrastive vowel length.