it's one major work of quite a reasonable size (word count/media presentation length depends on which form you choose, whether short story, poetry, multimedia, film, speech, etc)
internal assessments (marked by school) include: Proposal (written assessment outlining your intentions in your major work), viva voce (oral, interview type assessment), report (explanation and justification of your major work). this typically marks the PROCESS of composing a major work.
external assessments (marked by the board of studies) include: Major work, reflection statement. this typically marks the PRODUCT itself. ie major work and reflection statement.
it is one major work, no other exam, and you're given around 9 months to work on it, but remember that the level of research, writing process, including editing, and composition of reflection statement mean it is quite a lot of effort. it's more work than you'd think for a 1unit subject. at least as much as a 2 unit subject.
benefits include: no exam, and usually handed in around the times of trials so you don't have to work on it up until the final exams.
for good examples of past works receiving top marks, look at Young Writers compositions.