The rules are set by PTV, not by a sitting member (whoever that may be).
In the PTV Fares and Ticketing manual, on page 13, it says:
"Where V/Line train services stop at railway stations in the metropolitan area, pick up and set down restrictions may apply. These restrictions are set out in V/Line timetables which may be viewed or obtained at V/Line railway stations and viewed online at ptv.vic.gov.au. The tickets of customers boarding or alighting at stations in contravention of these restrictions are not, or cease to be, valid for travel on the relevant V/Line train service."
On looking at the Bendigo timetable on the PTV site, all Up trains stopping at Sunbury are marked with a D symbol, which means "Drops off passengers only".
It would seem from this that suburban passengers may not board an Up V/Line train at Sunbury, and if they do, they would not have a valid ticket (and could be penalised). It would also seem that a conductor who tries to prevent such boarding is not playing up, just doing the job.
If suburban passengers ARE in fact allowed on, as has been suggested, then the above section of the manual needs to be rewritten, and a new directive needs to be circulated to all concerned, so that there will be no doubt in anyone's mind - both employees and passengers. Alternatively, the D symbols need to be removed from the timetables, and station announcements such as "Do not board this train" need to be stopped.
Clearly PTV must do the right thing and take away the U and D next to Sunbury in the timetables - State government, PTV and V/Line said the following: "Sunbury can still use the V/Line service to/from Melbourne go to Sunbury.