If BFB is applied at a normal rate, I'd have thought any worm repellent qualities would disappear pretty fast as it rots.
NZ flatworms are a more serious problem. Have you got absolutely no worms, Greenstar, or only a few? The flatworms appear to like damp, clayey soil, and in such conditions they do wipe out earthworms. In drier conditions and sandy soils, earthworm populations can recover, or the flatworms can die out. Digging makes the problem worse.