Hi Beth I had a look on the archives site that you mentioned for Albert Parker when he enlisted and the last address was in graceville, brisbane in 1967 so he was alive then. Went back and checked 1922 electoral roll looking for alexander and robert i have found this in graceville, brisbane ALL THESE PEOPLE LIVE IN OXLEY ROAD, GRACEVILLE SUBDIVISON SHERWOODDIVISION OXLEY
Herbert Thomas Parker occupation storeman packer
Harriett Isabella Parker occupation home duties
Alexander Parker occupation grocer
Ada Mary Parker occupation home duties
I have electoral rolls can cut and paste them for you to check my email is email@example.com
I checked the cemetries online in brisbane and there is no alexander but a few alberts but i dont think the age matches up for yours.
Will keep hunting for you and i live only 30minutes to pomona so can take a drive and check cemetrys for you not a problem