They will have completed the work by the time we get there.

The work will be completed by the time we get there

The work will have been completed by them by the time we get there

The work will have completed by the time we get there

The work will have been completed by the time we have got there

All of the above

Explanation

Only the finite verbs (verbs that show or decide the tense of the whole sentence) undergo voice change. Here ‘will have completed’ is the finite verb.

(Incidentally, the verb ‘get’ in the subordinate clause ‘by the time we get there’ is intransitive in nature. It has no object in this context.)