It is likely that this is an adult bird that has returned to supervise or collect its chick and prepare for the northward migration and the magpie larks were defending their foster chick that they have raised.
Appreciate your insight into the behaviours of the cuckoo and magpie-larks. Both the Channel-billed Cuckoo and Koel have been more vocal over the past week or so and I had wondered why. Thanks.