This paper investigates whether colloquialisation – a stylistic shift by which written genres come to be more similar to spoken genres – has played a role in the endonormativisation of the grammar of Australian English, a variety which has long been noted for its penchant for colloquialism. The study tracks changes in grammatical colloquialism from the early 20th century against the historical backdrop of the progressive decline in Britishness in Australia and the pervasive effects of “Americanisation”. The data are derived from a suite of parallel Brown-family corpora representing British, American, and Australian English of the 1930s, 1960s, 1990s and 2006. Multivariate techniques are used to delimit 26 “colloquial” and “anti-colloquial” grammatical features from a set of 83 potentially relevant features, and to examine changes in their frequencies between 1931 and 2006, in the three varieties, and across the three major genres of fiction, learned writing and press reportage.