SCHEMA:
CREATE TABLE table_name_66 (population_2006_census INTEGER, local_government_area VARCHAR, rank VARCHAR)
USER:
What was the 2006 population for the local government area of alexandrina council whose rank was smaller than 19?

QUERY:
SELECT AVG(population_2006_census) FROM table_name_66 WHERE local_government_area = "alexandrina council" AND rank < 19