The ACN is a nine digit number with the last digit being a check digit calculated using a modified modulus 10 calculation.
author Byron Adams <byron.adams54@gmail.com>
author Daniel O'Connor <daniel.oconnor@gmail.com>
see http://www.asic.gov.au/asic/asic_infoco.nsf/byheadline/Australian+Company+Number+(ACN)+Check+Digit