Check out this site: You'll be surprised at how many, where, and what magnitude, earthquakes occur along the Eastern Seaboard of Australia....
http://www.esands.com/Earthquakes.html
Simplisticly, while the majority of the world's earthquakes are concentrated along the edges of tectonic plates (eg like the trench just east of NZ) , the shifting along plate boundaries also causes the fault lines across the plate surfaces to build up pressure: these then have to move to reduce those forces. There are quakes happening all the time (which is good: relieves the pressure in small increments) , but they are mostly below the threshold of being 'felt' by humans.
Clear as mud, eh?
Mike C