How do I carry out the Year 12 rollover process in Canvas for NSW?
Sentral creates Terms for each academic year by default. The Sentral integration includes a field to create a Year 12 Term separate to the other Terms. Simply name the Term so that it is easily identified as the Year 12 Term:

Once this has been created, go into Canvas and set the Term dates as appropriate for those students. For example, for students who will be doing their HSC in 2020, you would have Term dates going from the start of Term 4 2019 to the end of Term 3 2020. Students will still be able to access their courses after the end of the Term in Canvas in a read-only state.
In the break between Term 3 and Term 4, stop the automatic daily sync for the integration and make sure to update your timetable to the new Year 12 classes. Once the timetable is updated, you can then press the Rollover Data button to clear the Sentral integration database:

After you have checked that the sync is sending through the new data, you can then turn the automatic daily sync back on in the Sentral Canvas integration settings.