The guard shouldn't have to collect any fares, all passengers should have a ticket before boarding because all stations should have sufficient facilities for them to do so.
Then I woke up........
TPE will start serving smaller stations like Marsden and Greenfield where it probably won't be viable to do anything other than install a single TVM.
The guard could always be given an assistant to help with revenue duties - on some TPE services that is already the case.