Dalai, we had capacity for 22L plus five bidons. The only time we carried that much - between Wilpena and Hawker - we still had about 8L left after two days out in 43C heat. We could have taken less, but I’d rather have 8L spare if we ended up stuck out there for a bit.
I’d recommend the Sea To Summit reinforced goon bag style water bladders for storage; they were excellent. Way better than the old MSR Dromedary (which always tasted like rank) and much lighter/smaller. Cheap too.