BHP do have negotiating power, but Olympic Dam is one of GDY's potential customers. It would be surprising if GDY let GRK get the rig before PTR (although this is pure, straight-out-of-my-head speculation).
BHP do have rigs suitable for shallow drilling, but nothing appropriate for the wells GRK is proposing, a fact Alan Knights agrees with.
GDY might just have this situation by the balls.