High Court interpretation does not alter the written words in the Constitution; it only applies the meaning of the text in the Constitution to the case brought before the court. An advantage of the High Court interpretation is that a decision must be made, which generally results in a change in the division of law-making powers between the Commonwealth and the states. However, if a case isn't brought before the High Court then nothing changes. I would give an example of a High Court case - i.e. Franklin Dam case. Discuss its' impact.
For a referendum, it's effective in that it ultimately can insert or delete words in the Constitution. Give an example of a successful referendum - i.e. 1967. As with the High Court interpretation, it results in a change in the division of law-making powers between the Commonwealth and the states. Both are effective in that sense, more-so the referendum process, I believe. Though it is an effective process, talk about the low success rate of referendums, and give one or two reasons why it is low. An advantage of the referendum process is that individuals/groups can initiate change (whereas in High Court interpretation, it can't) and may ultimately result in a successful referendum. The obvious disadvantage being that referendums have a low success rate.
8 marks is a lot for a question like that. I don't know if what I've said above actually helps. That's just how I would approach the question.