Your bridging visa A will activate when the visa you hold expires.
You can leave Australia on a bridging visa A, but you cannot return.
To have entry rights you need a bridging visa B, for which you may apply from a bridging visa A, even if your bridging visa A is not active.
We already applied for PR (887). At present I am on 489 (NT Sponsored) visa which expire on May 2020. I just want to know that can I go to overseas before May 2020 on same visa or I have to apply bridging visa B.