For more than 20 years, the MacArthur-Bates Communicative Development Inventory (CDI) and its adaptations for languages other than English have been used as reliable measures of infants' and toddlers' early receptive and productive vocabulary size. This article introduces the OZI, the Australian English adaptation of the MacArthur-Bates CDI, now normed for 12- to 30-month-old children. The findings of two studies are presented: (1) a comparison study that demonstrated that toddlers (N = 64) acquiring Australian English (24- and 30-month-olds) obtain higher productive vocabulary scores on the OZI than the MacArthur-Bates CDI; and (2) an OZI norming study that included 12- to 30-month-old Australian infants and toddlers (N = 1496). These studies provide (i) evidence for the greater applicability of the OZI for infants and toddlers learning Australian English and (ii) productive vocabulary acquisition norms for Australian English for ages 12-30 months, a research and diagnostic tool highly anticipated by researchers and clinicians around Australia.