The IUCN Red List criteria are a globally accepted method of assessing species extinction risk, and countries around the world are adapting these criteria for domestic use. First, we compared trends in IUCN Red List criteria used in threatened plant species listings in Australia and globally. Second, using the state of New South Wales (NSW), Australia, as a study region, we conducted two complementary analyses: (1) An assessment of ~ 5000 currently unlisted NSW plant species against the thresholds for the geographic range criterion (Criterion B) to identify species which may require full assessment; and (2) A rapid assessment of currently listed threatened plant species, applying the IUCN Red List Critically Endangered thresholds for all criteria, to identify species likely to be at the highest risk of extinction from further decline. Impacts on these species could be considered to be "serious and irreversible impacts" (SAII). Geographic range size was the most common criterion used in Australia and globally for plant listings. Our assessment of unlisted NSW plant species revealed 92 species (75 endemic to NSW) met the geographic range thresholds for Critically Endangered. Our rapid assessments of currently listed NSW threatened plant species identified 53.5% as having an extremely high risk of extinction should further decline occur. Of these, most were flagged under Criterion B (88.8%). Geographic range and the other IUCN Red List criteria thresholds for Critically Endangered provide a useful framework to identify species at an extremely high risk of extinction from ongoing decline.