I still believe this is an Australian Darter. 1 - the bird has red feet whereas the little pied cormorant has black feet. 2 - the 2nd & 3rd photos show the white line through its face and its face is not fleshy and yellow. Also, see sighting #4713343 for a similar looking bird.
Thanks Don. It's certainly a jungle out there. Last year's attempt by the kookas' failed when a brushtail slipped in and broke the eggs during the short period between the end of the female's overnight incubation shift and the male's arrival. Both birds entered the box and confronted the possum for many minutes to no avail. They didn't return last season and, while they have since inspected the box from time to time, they show no indications of using it in the coming season. On the other hand, a pair of wood ducks have visited every day since mid-June and show every sign of breeding there. Rather than disturb them now, we'll leave them to take their chances for this season, and defer any predator-protection modifications until next year. PS: the kookas did breed successfully last year, although I don't know where they nested. Presumably they will re-use that nest in the coming season, assuming it is still available.
It will eat the nestlings if the Kooka does not defend them continually. Which is good for baby sugar gliders. If you prefer baby Kookas, place collars above and below the box, and cover the sides, front and lid of the box with somethig slippery like polycarbonate sheeting. There is a lot of good info on the nest box tales web page about reducing predation by Brush Tailed Possums and Sugar Gliders. The treatment also reduces predation by Roof/Black/Ship Rats (Rattus rattus).