What is the passive voice of haven't they done it?

Hasn't it been done?

The example is in Preset Perfect, so I'll keep the explanation in this tense, as well.

In the active voice in English, first you say who has performed an action (they), then what action they have performed (haven't done, which becomes split in a question), and finally you also say what or whom the action has affected (it).

In the passive voice in English, first you say what or who has been affected by an action (it), then what action has been performed, using the verb be in a given tense and the Past Participle form of the action verb (hasn't been done). You often leave out the person (or animal, phenomenon, etc.) by whom the action has been performed as that fact is either unimportant or unknown.