They're big campsites, there's plenty of space. You won't miss out.
It looks like St Bathans is a $5 "donation" probably cash paid into a box or something. (Tbh I don't remember paying this last time, woops)
Piano flat is a doc site, $10 per night. They prefer you book online these days, i think locals jack the cash boxes.
https://www.doc.govt.nz/parks-and-recreation/places-to-go/southland/places/waikaia-forest-conservation-area/things-to-do/piano-flat-campsite/
@- i5oogt - has numbers for the farmhouse