If it can get in your drinking water, you'll probably find it in stormwater too, and if ingested this protozoan will cause diarrhoea and stomach cramps. Giardia is also occasionally found in stormwater. Worms tend to use a snail vector which is not common to Australia. Many kinds of algal single-celled life exists but have never caused trouble. In theory one could concievably get anything from a sewage overflow into a drain.
Cuts are common when one falls over, and people have occasionally ingested runoff unintentionally.
Trouble is, it appears as a cold, rapidly degenrates into pneumonia, and then kills you due to fun things like hepatic failure. You have to smash it with antibiotics during its incubation period, after which time it is too late and you tend to die. One never can tell when it will happen. We have had deaths through cerebral annuerism, suicide, motorbike and mountaineering accidents but our safety record is so far unparalled. Thus I suggest prior immunization.