Forest Ravens can dig up huge areas of turf. The one pictured is only medium size by their standards. On our eight hectare property, the two resident ravens excavated eightor ten patches like this one over several months.
They work methodically, starting at a single point and working around the perimeter, gradually spiralling out. They will work on one site for a half hour or so; then a little later they will be over on the other side of the garden working on a different one.
Now and then the Black Currawongs re-turn a few sods in the hope of finding something interesting underneath one.
Years ago I saw similar large patches of turned-over grassland in the New South Wales highlands and, at the time, assumed that it was feral pigs. (I may have a picture of this somewhere.) But perhaps that too was Forest Ravens.