You can find your previous BMS ID via ATO's online services portal.
1. Log on to your ATO's online services.
2. Go to Employees and click on STP reporting.
3. Expand your STP reports by clicking on the dropdown arrow.
4. You will find your Business Management software ID (BMS ID)
Your tax/BAS agent can find your BMS ID via ATO's online services as well under the Business tab.