In general the Australian White Ibis has a Black hood rather than a black collar length neck and the Sacred Ibis does not have Red marking at the top of the neck at the back of the head,
Also when in flight the Australian White ibis as Black tips to the primary feathers on the wing where as the Sacred as much deeper colouring all through the primaries.
Now that you point out the differences I can see them but they do look really similar. At least you won't have to worry about telling the difference in the wild.
formerly the Australian white ibis (Threskiornis molucca) and the Asiatic black-headed or Oriental white ibis (Threskiornis melanocephalus) were included in the same species as sacred ibis (Threskiornis aethiopicus). You'll see especially in older books the first two species being referred to as sacred ibis, because at the time that was correct. Some authors still maintain they are all only subspecies.
Generally speaking, in the wild there isn't any possible confusion except in Taiwan where African sacred ibis are well-established following zoo escapes about fifteen years ago. These are the only one of the three species resident on the island, but Asiatic black-headed ibis are also natural vagrants to Taiwan.
In Singapore, Jurong Bird Park used to have had free-flying Asiatic black-headed and African sacred ibis. Neither of these are currently free, but in the past individuals of both species were sometimes seen in random wetland areas on the island.
