Tripneustes species are known for their boom-and-bust population dynamics. In New South Wales (NSW) and New Zealand the subtropical to temperate endemic T. australiae occurs on rocky reefs and sea grass meadows. In aggregations, T. australiae can strip these habitats of plants through grazing. Tripneustes gratilla, a tropical vagrant is also present, but far less abundant. We used the Reef Life Survey data to follow the density of T. australiae from 2008-2023 at three locations, Solitary Islands, Lord Howe Island and Sydney, encompassing its main latitudinal distribution. On a roughly decadal scale T. australiae appears in conspicuous numbers followed by a marked decrease in density. The broad size range across several sites indicate that cohorts have recruited over recent years. Tripneustes australiae is common at the Solitary Islands and started to appear in Sydney following mass mortality of resident sea urchins during the 2022 floods. While conspicuous, numbers of T. australiae along the coast remain moderate. At Lord Howe Island, T. australiaeincreased in 2022 and is now in outbreak densities resulting in creation of barrens-like habitat. The drivers of the boom-and-bust population dynamics of T. australiae are unknown but are likely related to climate and are important to understand.