Regarding the first link given by iains, what on earth are a group of people at Australian, Welsh and German universities doing publishing in the journal of the Siberian Federal University? My guess is that they couldn't get it published anywhere that we have heard of. That paper proposes that there was an impact of a 1.6km diameter object in 300m of water, in AD1500, just south of New Zealand. Sorry but I think that there would be other evidence for that. A later paper by James Goff et al. dismisses that hypothesis. Unfortunately you can't read any more than the abstract of that paper without paying Elsevier loads of money.

Don't get me wrong, impacts are a threat, and NASA, ESA and others are right to take the measures they are to catalogue the threats, predict possible impacts, and develop mitigation strategies. But in my view the geological hazards are greater, and if I lived on the east coast of Australia it would be Taupo and related volcanos which I would worry about more. Not to mention Toba on Sumatra.