When production ceases, offshore oil and gas wells are taken out of service which involves safely plugging, abandoning and usually removing most of the associated subsea equipment. This process must consider impacts to marine ecosystems to meet regulatory requirement and for best environmental practice. However, there is a paucity of information globally on the ecosystem value of these structures, despite the many thousands that are installed throughout our oceans. This study provides the first assessment of fish assemblages and habitats formed by colonising invertebrates on oil and gas wellheads and associated infrastructure in depths of 78–825 m on the north west shelf of Western Australia. Video footage was obtained from Remotely Operated Vehicles deployed by industry on 25 wellhead structures, with six surveyed in each of four distinct depth zones: 78–85 m, 125–135 m, 350–395 m and 490–550 m, and one in 825 m depth. A total of 7278 individual fish from 60 species and 35 families were observed. Commercially important lutjanid (snapper) and epinephelid (grouper) species were common and most abundant on well infrastructure to depths of 135 m, but were absent in depths > 350 m. An as yet unidentified species of roughy, recorded here as Gephyroberyx sp. was the most common fish species observed on well infrastructure in depths > 350 m. Two speckled swellsharks (Cephaloscyllium speccum), believed to be endemic to north-west Australia, were observed for the first time in situ. Numerous fish species were observed at depths beyond their known limits and two IUCN vulnerable species were recorded: the grey nurse shark (Carcharias taurus; 135 m depth) and the round ribbontail ray (Taeniura meyeni; 78 m depth). Fish assemblages and colonising invertebrate habitats present on wellheads and associated infrastructure were strongly influenced by depth, age and height of the structures. Older, taller wellheads in depths < 135 m possessed greater abundances of groupers, snappers, site-attached reef species, and transient pelagic fish species. Beyond 350 m depth, the number of species and total fish abundance declined markedly, as did the percent cover of ascidians, black/octocorals, sponges and Gorgonocephalidae (basket stars) observed growing on the infrastructure. Deeper structures were characterised by an abundance of Gephyroberyx sp. and, while these structures had less colonising invertebrate coverage in general, crinoids (490–550 m) and crustacea (barnacles; 350–395 m) were dominant at these depths. With very little known about marine ecosystems in depths > 100 m, or about wellheads as a type of subsea structure, this study demonstrates the ecological value of ROV footage obtained during industry operations and is indicative of the importance of subsea oil and gas infrastructure as a habitat for fish, and potentially as structures with value to fisheries.