When I was a final year undergrad, a housemate of mine applied for a PhD in her Department, and had to give a presentation to a few members of staff and post docs. However, it was helped by the fact that she knew a couple of the staff, and her presentation was merely a rehash of her dissertation viva, so it wasn't too bad for her. Presentations at interview seem rare, but I presume they're looking for someone who is good at speaking to a group. Good luck!