The AusDigital Billing Specification is based on OASIS UBL2.1 billing semantics and the ADBC implementation guides.
Specification: https://ausdigital-bill.readthedocs.io/
Community discussion/support: ausdigital.org
Open source reference implementation: https://github.com/test-point/testpoint-bill/
Test suite: https://github.com/test-point/testpoint-bill
Live test endpoint: https://bill.testpoint.io/