During 1976-78, 212 Australasian Harriers (Circus approximans) were trapped and individually marked, and a total of 220 retraps and 319 resightings were made. During two breeding seasons the population density averaged one bird per 50 ha. Seven territories averaged 31 ha each, and the home ranges of four pairs averaged 900 ha each. Some of the behaviour and displays described have not been previously recorded for the Australasian Harrier, including territory-boundary display flights, border patrolling, eviction of intruders, nest inspection, courtship feeding, copulation, and post-fledging behaviour and dispersal. Also described are display soaring, display diving, feeding at plucking stations, aerial food passes and the post-hatching parental division of labour. Nineteen pairs fledged an average of 1.0 young per nest site and 1.8 young per successful nest. Birds observed breeding at Pukepuke Lagoon for a second consecutive season were more successful than new arrivals. Two cases of polygyny were observed.