It depends on the topic (essay question) and the line your essay is taking on it.
Your opening sentence should
1. Refer to the key terms (or issues) of the question (including the HSC verb), if possible in your own words
2. Indicate the line you are taking on the question
The rest of your introductory paragraph should preview the main points in the argument you will be presenting in the main body of your essay.
Eg
If the question was:
"Discuss how far Tacitus is biased in his depiction of Germanicus"
your opening might be
"While it is possible to trace a basis of historical truth in Tacitus's portrait of Germanicus in the Annals, there seems to be little doubt that the historian presents Germanicus essentially as a dramatic foil to Tiberius, heightening the young patrician's good qualities and achievements to more effectively cast Tiberius in the character of a tragic tyrant. The main ways Tacitus does this are w, x, y and z, as Wilhelm Frankenfurter has argued; although, as I shall discuss, Professor von Brainstorm has criticised this interpretation because of a, b and c..."
and so on; then go on to discuss in detail in your main body w, x, y and z, and a, b and c.
Of course, your question is probably different from this & you may take a different kind of line.