Thanks everyone for your replies. I have a copy of my Australian birth certificate, which lists one of my parents as having been born in Australia, thus confirming my citizenship (no need for a citizenship certificate). As to why I would need a passport, I couldn't think of a reason myself, hence why I asked. The fact that the 40SP form specifically asks for passport details, rather than simply asking for proof of citizenship, was what was worrying me, but I am hoping that if I just include a copy of my birth certificate and a few other forms of ID, that it should be fine.