This is normally found on dwelling sites 25+ years old, but it is not uncommon to come across modern dwellings with sewage systems discharging to a watercourse. Most dwellings connected to a water course likely had a soak hole at some stage that possible blocked up and lead the homeowner to connect it to a watercourse to relieve the situation.
Discharge from a septic tank is very dangerous and contains fecal coliforms containing E. coli and Kryptos Sporidium among billions of other harmful bacteria. You should have your drinking water tested and until the issue is rectified you should fence off any area children and pets could make contact with the waste water.
Firstly a site suitability assessment will be completed at design a new system taking into account all the unique features on the site, followed by installation.