"I am going to go to Australia next year."
I wouldn't say it's wrong. If I plan on going swimming next week I would say " I'm going swimming next week", rather than " I plan on going swimming next week". Your translation is more literal and should be accepted, but it's not necessarily more correct in it's spirit. Persosonally I'm not a fan of literal translations since it only makes people try to translate everything literally while speaking Japanese leading to weird non-understandable sentences. It's better with variations, making the students think about different meanings of the same sentences.