[aiham]

15 minutes was part of the definition for a casual contact 2 months ago, a lot earlier than the app and the social distancing rules.

https://www.health.gov.au/sites/default/files/documents/2020...

[ggm]

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.