What is the ISO code for Australia?

ISO 3166-1 alpha-2 (2 digits) for Australia:

The ISO 3166-1 code for Australia is: AU

ISO code country for
Each country, or geographical entity, or province, or department, etc. has been awarded codes defined in ISO 3166 by the International Organization for Standardization (ISO).
ISO 3166-1 alpha-2: two-letter codes are used for many applications, including top-level domains for each country.