Connect to zone and determine the auto generated password for postgres user:
cat /var/svc/log/system-zoneinit\:default.log | grep PGSQL_PW
document the result and log into postgres with the following command, entering the password when prompted:
[root@psql ~]# psql --user postgres
Alter the postgres role's password:
postgres=# ALTER ROLE postgres UNENCRYPTED PASSWORD 'new-password';
Now exit (\q) then try to log in with the new password.
In my case I was setting up PostgreSQL for testing out Zabbix monitoring system. So I did the following as the postgres user:
postgres=# CREATE USER zab UNENCRYPTED PASSWORD 'zabby' postgres=# CREATE DATABASE zab OWNER zab;