"Being from Australia, you're no doubt familiar with eucalyptus, and I know of no faster tree. They do get big, however, so that may be an issue.
Willows (Salix) are also used for screening. Very fast, and don't get quite as big (but still not tiny).
Evergreens such as Italian cypress or arborvitae are excellent screens. Not as fast as the above options -- will take a several years to become an effective screen -- but otherwise are highly recommended screening plants."