The crescent honeyeater is arboreal, foraging mainly among the foliage and flowers in the understory and tree canopy on nectar, fruits and small insects. It has been recorded eating the honeydew of psyllids, soft scale and felt scale insects. It feeds primarily by probing flowers for nectar, and gleaning foliage and bark and sallying for insects. While regularly observed feeding singly or in pairs, the crescent honeyeater has also been recorded moving in loose feeding flocks, and gathering in large groups at productive food sources. A study in forest near Hobart in Tasmania found that the crescent honeyeater's diet was wholly composed of insects during the breeding season, while nectar was a significant component during winter. Insects consumed included moths and flies. Tree-trunks were the site of foraging around two-thirds of the time, and foliage a third. It fed on nectar as plants came into flower in the autumn and winter, and then foraged in Tasmanian blue gum (Eucalyptus globulus) during the breeding season in spring. The flowering of royal grevillea (Grevillea victoriae) over summer in subalpine areas in the Snowy Mountains attracted large numbers of crescent honeyeaters. It feeds intensively when sources are plentiful and, when feeding on flame heath (Astroloma conostephioides), it was recorded visiting an average of 34 flowers per minute. Other plants it has been recorded visiting include a number of Banksia species, waratah (Telopea), tubular flowered genera including Astroloma, Epacris and Correa, mistletoes of the genus Amyema, and eucalypts in the Mount Lofty Ranges in South Australia. In Bondi State Forest it was also recorded feeding at cluster-flower geebung (Persoonia confertiflora), native holly (Lomatia ilicifolia), tall shaggy-pea (Oxylobium arborescens), silver wattle (Acacia dealbata) and blackthorn (Bursaria spinosa). Local differences in flower foraging patterns have been observed in South Australia; populations on Kangaroo Island forage more often at Adenanthos flowers than those in the nearby Fleurieu Peninsula, while the latter forage more often at eucalypt blooms, and at a higher diversity of plants overall.