Reply I received from a New Zealander when I asked yesterday:
"Farewell spit is the problem, its a long narrow stretch of sand that is at the northern end of a large bay and the water is very shallow there to the south of that spit. There was big tide...they get 5m tides there I think, and as the tide comes in the whales follow it..and they think they are heading to open water..then the tide drops fast and they get trapped...and their distress calls brings in their mates...
They refloated them..the volunteers..but they get confused by the shallows and re beach."
So Devil's right, natural event.