The weedy potential of deliberately introduced plants has been a growing concern in Australia since the late 1980s. Although introduced plants are critical to Australia’s agricultural and livestock production, many species that were praised in the past are now declared agricultural and environmental weeds. Nevertheless, weeds researchers appear largely ignorant of the magnitude and intent of plant introductions for agricultural purposes as well as the legacy of unwanted plants. Across more than 70 years, Commonwealth Plant Introductions comprised 145 000 accessions of more than 8200 species. These species include more than 2200 grass (Poaceae) and 2200 legume species (Fabaceae sensu stricto), representing about twice the indigenous flora in those families and about 22 and 18%, respectively, of the global flora of grasses and legumes. For most of the 20th century, these and other introductions supported research into continental-scale transformation of Australian landscapes to support greatly increased pastoral productivity in order to achieve policy goals of maximum density of human population. This paper documents some of the scientific developments and debates that affected the plant-introduction program. We argue that recent developments in weed science and policy need to be informed by a better understanding of plant-introduction history.