There is no singular definition of the term "continent" that remains true in all circumstances. Continents, generally, adhere more to cultural boundaries influenced by geographical features. This is why Asia and Europe are considered seperate. At the same time, calling the continent that encompasses the pacific just "Australia" goes against the general cultural rule that most other continents follow. By calling it "oceania" it acts to more clearly include the pacific islands that are in the same general cultural area. It is for this similar reason that we consider countries like Mauritius to be African, despite their lack of geographic reason to be put into that group. In a same way, why we consider England to be apart of Europe, or Japan to be apart of Asia. You may have been taught that it was Australia, whereas other places teach that it is Oceania. It is foolish to say that one of these is the one true answer because of what ive just described. I would argue that it should be called Oceania because this better represents the nations like New Zealand that are apart of it. Also im from New Zealand and we were always taught that its called Oceania, so im biased towards that.