Domestos is septic safe.

I do sometimes use White King though, as it is the only thing that can clean our old bath (for from reason domestos sends it bright orange). I just use a little as I can get away with and wash it down with as much water as possible (whilst at the same time not trying to waste the precious tank water).

If you do manage to stuff your bacteria, you can buy something to get it right again. Think it was just a sachet from Bunnings.