Banjul, Gambia is also missing. And Canberra shouldn't be on there because Australia is a continental landmass, not an island in the sense the word "island" is usually meant. Check out some of the more popular island tests on here; you'll find that hardly any treat Australia as an island.
Although I usually treat Australia as a continent, I would not call Canberra "wrong" here just because danem17 chose to treat it as an island. I see 82% answered "Canberra" here so I don't see much confusion,. You can legitimately argue it either way, one of several gray areas in geography quizzes that make them interesting.
Just because a lot of people were able to guess a wrong answer doesn't make it right. Australia is definitively not an island. If you accept Australia as an island, then every continent is an island, and every capital in the world is on an island.
I agree with sumguy. Canberra never occurred to me as an answer because it never occurred to me that anyone could possibly consider a continental landmass to be an island.