They are scavengers to a large extent. They rob birds nests and eat smaller reptiles as well as carrion. I once camped with my family on a bush headland next to a burnt out shack. The old water tank was off its platform and we could hear one of these lizards (we call them goannas) scratching around inside the tank. He was quite big.
Later I saw him climbing a tall eucalypt with a rozella in his mouth. I presume he found it dead because I cannot imagine him moving quickly enough to capture a parrot.
He was no threat to us, leaving us alone and, being sensible, we left him alone. However on one occasion we did offer him a couple of hard boiled eggs which he took quite delicately from our hands. I found the encounter quite thrilling.