As far as I know, there are two ways of writing the passive form in this case. It's the "impersonal passive" and verbs like think, believe, say, report, know, expect, consider, understand, are used in these constructions.
Active: "They believe that she has been telling the truth".
Passive 1: "She is believed to have been telling the truth".
Passive 2: "It is believed that she has been telling the truth".
The passive verb form of the present continuous tense to have been + [verb+ing]
is a weird construction and rare in the passive:
Active: John has been telling me for ages that you are leaving.
Passive: I have been being told (by John) for ages that you are leaving.
Advanced Grammar in Use, Second Edition, Page 224, Cambridge Presshttp://passivecausality.weebly.com/pers ... tions.html