Not quite. G-NAF is a government owned enterprise, separate to the privitised but government body of Australia Post.

G-NAF is the equivalent to the UK's National Address Gazette.

It's a separate body of data, that sometimes disagrees with the "source of truth" that is Australia Post, and all the post systems that rely upon them.

For example, it took two years for G-NAF to notice that Winter Valley, Victoria, is not within 3356, but actually has its own brand new post code of 3358.