The 18-digit numbers are created as follows:

GS1 Company Prefix: This will be allocated to you by GS1 Australia and will vary in length from seven to nine digits. The physical builder or the brand owner of the logistic unit is responsible for the GS1 Company prefix used in the SSCC and makes the SSCC unique worldwide but does not identify the origin of the unit.
Serial reference: This is assigned by you, and we recommend that you begin with zeroes and increase the count sequentially. The structure and content of the serial reference is at the discretion of owner of the GS1 Company Prefix to uniquely identify each logistic unit. We recommend that this number should not have any meaning in it.
Check digit: This is calculated using the first seventeen digits of the number. A check digit calculator is available.
The GS1 Application Identifier (00) indicates that the GS1 Application Identifier data field contains an SSCC). The extension digit is used to increase the capacity of the serial reference within the SSCC. It is assigned by the company that constructs the SSCC. The extension digit ranges from 0-9.
SSCC Fact sheet
Logistics Labels Fact sheet
Identification FAQs