Missions in Risk -- is Australia really an island?
Australia is not an island.

The rules for the 2003 edition define an island as follows:


  A mission may require you to control an island. An island is a territory completely surrounded by water and only connected to other territories by sea-lines, for example Indonesia.


See page 18 of the rules here: http://www.hasbro.com/common/instruct/Risk_2003.pdf