I have a table which has all towns and States and postcodes (zipcodes)
for Australia.  Many duplications of town name occur as there can be
more than one town in different states or even same town name in one
state but different postcode.

No problem , the data is already there.

However I would like to be able to add non Australian Names but ensure
there is no duplication on adding.  I cant use Town name as Indexed ,
no duplicates (because of duplication mentioned above) and need a
method to have a combination of State, Town and Postcode or Twon and
Postcode not allow for duplicates.
Any help appreciated
Steve