Well, an event of that magnitude would have a much more widespread impact than just taking out one continent. An impact that took out for example, Australia would cause havoc throughout the world. The whole world would ring, causing massive earth quakes world wide. This would be accompanied by massive fall out from the ejecta from the impact. This is the sort of disaster that might indeed kill off the human race. Extinctions would occur on a massive scale. However, we are talking about an impact by a planetary scale body, such as the one that is believed to have created the moon. There are no bodies of that size in the inner solar system that could impact the Earth. It might be possible for a body to be deflected into the inner solar system from the far reaches of the solar system, but it unlikely even in that case that it would impact the Earth.
A more credible threat would be a smaller asteroid, something the size of the one that is believed to have caused the extermination of the dinosaurs. It is estimated that that one had a diameter of 10 km. It is possible, although improbable, that we could be hit by one that size. In that case we might have some extreme weather over the entire Earth, but a good sized part of the human population would probably survive. Check out the report on Wikipedia
Chicxulub crater.
And for super-volcanoes I think we would have approximately the same problems as for the massive asteroid impact.
Keeping in mind that this is my own personal and not necessarily well informed speculation.
Bill Gill
