Semi-arid floodplains are dynamic ecosystems characterised by alternating wet and dry phases that support distinct vegetation communities. However, river regulation has reduced flood frequencies and increased the duration of dry periods in many semi-arid floodplains, impacting floodplain vegetation. Environmental water is increasingly used for floodplain restoration, making it critical to understand the potential for degraded floodplain vegetation to respond to flooding. We used a large, unplanned flood to investigate how floodplain vegetation communities with reduced flooding histories can transition between dry and post-flood phases. The study area encompassed 67 sites in four floodplains on the Murray River in north-west Victoria: Belsar-Yungera, Hattah Lakes, Wallpolla Island and Lindsay Island. The sites included treeless and woodland vegetation across a gradient of time-since-prior flooding and past flood frequency, with some sites unflooded for many decades. Our first survey was during a dry phase, six months before the study area was inundated by widespread La Niña-associated floods. We resurveyed the sites after flood recession. The dry-phase vegetation community was dominated by terrestrial shrubs and many sites had low or no live plant cover. Flooding caused marked changes in community composition, including near-complete species turnover. Species richness and cover increased, with the post-flood community dominated by short-lived herbs with persistent seed banks, including many threatened species. The dominant floodplain shrub, Duma florulenta, decreased in cover in woodlands but was maintained in treeless sites. A small subset of repeatedly sampled sites after the flood indicated a return towards the pre-flood dry-phase community. These responses suggest that large natural floods can promote vegetation recovery, even at sites with little or no dry-phase vegetation. In turn, this indicates the potential for degraded floodplain vegetation to respond to managed floods, although additional interventions (e.g. revegetation and grazing protection) may be required to achieve target cover, diversity, and community composition.