1. The police saved us.
This is the simple past tense, and so must remain a simple past in the passive, i.e.
1a. We were saved by the police.
The other sentence however is a present perfect:
2. The police have saved us
When you transform it into the passive, therefore, you must use a present perfect passive:
2a. We have been saved by the police.
Let me know if you need any more help with this!
All the best,
Student or Learner