Migration and overwintering of Monarch butterflies (Danaus plexippus) in south-eastern Australia is reviewed. Overwintering, non-reproductive aggregations of Monarchs were first reported in the 1930s and 1940s from near Sydney, New South Wales and Adelaide, South Australia but scientific studies on overwintering and migration did not occur until the 1960s–1980s. During the early 1960s, overwintering populations ranged from 15,000–40,000 individuals at sites SW of Sydney. In the late 1970s overwintering populations in the Sydney area comprised only 400–3,500 individuals, a decline of > 90%. By 1984, 21 sites harboring autumn or winter populations had been reported from the Sydney area, the Hunter Valley (∼ 100 km NNW of Sydney) and the Mt Lofty Ranges near Adelaide. Overwintering sites in New South Wales appear to be populated by autumn migrants originating from higher elevation areas to the SW and W of Sydney, the Hunter Valley and lower mid-north coast. Migration and reproductive dormancy in late summer and autumn Monarchs are induced by cool (< 18 °C), overcast conditions post-eclosion. All overwintering sites are characterized by the presence of Milkweed (Gomphocarpus fruticosus L. (R. Br.)). Additional overwintering sites are reported from New South Wales (8), South Australia (1), and for the first time in Victoria (2) and on Flinders Island (Tasmania) (1), providing a total of 33 sites in SE Australia currently or historically known to harbor clustering or overwintering populations of Monarch butterflies. Populations reported at the new sites ranged from <100 – >2,000, suggesting no further radical decline in Monarch numbers since the late 1970s. Observations on the phenology, size and behavior of transient (2013) and overwintering (2017) populations of Monarchs at a new site 275 km N of Sydney, New South Wales, are reported. Overnight clustering of Monarchs on Milkweed host plants (G. fruticosus) at this site is documented. The annual status, size and phenology of Monarch populations at overwintering sites in south-eastern Australia is unknown and must be a priority for future research and monitoring. The non-native status of the Monarch and the Milkweeds it uses, has in the past not favored concern and conservation of this iconic species in Australia.