Since European settlement of Australia, the dry open forests and woodlands of central Victoria have been extensively cleared and most large trees harvested, resulting in a decline of arboreal mammal populations. The Powerful Owl, which was formerly reliant on these prey species, still persists in the region but at very low densities and uncertain viability. Previous research has shown that Powerful Owls select home-ranges with more large trees and hollows than the forest at large, but the amount of such habitat that is required remained undefined. Four adult Powerful Owls (two males and two females) from four pairs occupying geographically separate territories in box-ironbark forest were radio-tracked over 1–6 months. Home-range size was much greater than previously assumed for this species (minimum convex polygon of 4774, 2896, 1770 and 1382 ha). Range-length was 5.7–8.9 km, and on average 5–12% of each home-range was used during a single night. Core foraging areas comprised many, typically small, patches scattered across the entire home-range. Selection of roosting sites was flexible and did not constrain spatial use of home-range, with 96% of roosts in very small to medium-sized trees, which are widely distributed. The finding of unexpectedly large foraging ranges suggests that enhancement of habitat quality and mammalian prey abundance in currently occupied home-ranges is the foremost goal for forest managers if a viable population is to be sustained.