Technically, they can ask you for anything, and not just limited to an interview. Letters, reports, poetry, a song, a website, anything. That said, I believe - as of when I did the HSC in 2004 - that it's highly unlikely they will ask for a poem or song. Poetry requires a technical precision that you won't be able to get in 40 minutes unless you absolutely know what you're doing or already have those skills, and it's difficult to perfect the rhythm and meter required for a song. nwatts may tell you otherwise on both counts, but don't shoot the messenger - I'm only repeating what I was told by a veteran marker of the English paper.