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Angie,If it's big (2.5-3.0cms), and black with brown margins to the various body parts, then you may have a Great Diving Beetle Dytiscus marginalis. If you're trying to fish it out use a net, they have strong jaws - if you succeed with that, you may in future have to deal with the larvae if it's laid eggs. Speaking from personal experience, they bite also! If one has flown in then there's always the possiblity that others could do so in future. Both adults and larvae are predators (anything up to the size of fish, newts, etc). Here's a link for identification: http://www.eol.org/pages/1171540Peter
The long horn beetles are quite common, the larger species are very good hissers.New Zealands largest species is the Huhu beetle Prionoplus reticularis , scares a lot of people when it is attracted to light. A lot of people eat Huhu grubs, they taste like peanut butter.Attached a pic-hope it works. edit by maggi: it does work but I'm replacing it with a jpg which is easier!