New Zealand, long recognised as a land where birds dominate the terrestrial vertebrate biota, lacked an informative fossil record for the non-marine pre-Pleistocene avifauna until the twenty-first century. Here we review recent research that alters the known diversity of the fossil Paleogene–Neogene birds and our understanding of the origin of New Zealand’s recent or modern biota. Since 2010, there has been a 50% increase in the number of described fossil bird species (now 45) for the pre-Quaternary period. Many represent higher taxa that are new or listed for New Zealand for the first time, including 12 genera (35 total), nine family-level taxa (18 total), and seven ordinal taxa. We also review recent multidisciplinary research integrating DNA and morphological analyses affecting the taxonomic diversity of the Quaternary avifauna and present revised diversity metrics. The Holocene avifauna contained 217 indigenous breeding species (67% endemic) of which 54 (25%) are extinct.