I'd love to know where 15 minutes exposed came from. Feels like a value imputed from a join over battery drain and usefulness. I thought five minutes made more sense. If you are 15 min within 1.5m of a stranger in most Australian states you're probably mildly in beach of social distancing.
You will notice in that guidance that if you subsequently fall ill as a casual (sub 15 min) contact they will need to talk to you for contacts tracing..
So that implies there is a health risk burden under 15 which needs to incur costs of contact tracing at which time this app cannot help.
