The yellow gape - the line between the bottom of the eye and the bill - tells you it's a juvenile or baby bird (the colour tells the parent where to put the food).
It seems to have a white rump (the bit above the tail, but below the main back), and the head appears to be turning reddish from the back which suggests it would have a reddish head when it's mature.
Therefore I think it's probably Fairy Martin (binomial Latin name Petrochelidon ariel, a relative of the Swallow).
I'm not an expert on Australian birds, but with the red beginning to show at the rear of the head, I can't see what else it could be. But there are Australian bird experts on BF who will be along to correct me if I'm wrong.
From Japan, my (Japanese) wife and I (British) have been to Cairns (several times), Brisbane, Sydney, Melbourne, Perth and Uluru in the last ten years. Adelaide and South Australia were next on our list (Darwin will have to wait until my wife retires because the good season is our summer, your winter, and she can't get holidays at that season).
But who knows in the current strange and unexpected situation when we will be able to get to Adelaide?
Still, we are well and healthy, and our friends and family, too. I hope the same is true of you. And visiting your country again is something to look forward to.