The platypus nesting burrow, where females lay eggs and rear their young, has not been well studied. We have little knowledge of its structure and the process of construction. This study aimed to investigate nesting behaviour of breeding females and to describe the structure and features of the burrow. We used infrared cameras to record behaviour of captive breeding female platypuses during the nest-building period, over nine years. After the young had become independent, we excavated 11 nesting burrows and mapped their structural features. Nesting behaviour was observed 7-15 days after mating and was an indicator of gravidity. Females invested an average of 8 h 18 min over 3.5 nights, gathering and transporting wet nesting material to their burrows. The nests were composed mostly of native mat-rush leaves. Nesting burrows varied in length from 3.2 to 10.4 m. They contained narrow tunnels, 'pugs' of backfilled earth, dead ends, multiple entrances and a chamber at the end that contained the nest. Appropriate nesting sites and nesting materials must be provided to female platypuses for captive breeding programs to succeed.