[This is achievement is earned by completing a championship that uses the "Aussie rules" filter, creating any other ruleset will not unlock the achievement. Thanks to https://www.trueachievements.com/xJohnGalt117x.htm for confirming.]
As soon as you have the ability to open championships (for me it was at my second festival) go to the blueprint, press RB until you reach "country" and select "Aussie rules". Complete the championship you just created (you may need to buy a new car) and unlock your achievement.drub4n
points out in the comments that you can use a championship a friend has created, as long as it uses the "Aussie rules" filter.CR4SHED
says going for this achievement in the co-op campaign will only unlock it for the host, any other members will not receive it.