Translation:I mostly eat fruits for breakfast.
It sure is in Australia! "Fruits" implies more than one variety, where "fruit" suggests (more or less) one kind. The fact that you use "fruits" over "fruit" places an emphasis on this variety. It may not be perceived as being as common as "fruit" but it is correct English and is still in-use. And "fruits" also appears equally correct in other contexts.
I'm from NZ, are you sure it's correct? Or just often used in a wrong way like "should of" in place of "should have". "I mostly eat fruit for breakfast" doesn't imply one kind at all, it's general and doesn't specify. If I wanted to specify multiple types of fruit I'd say "types/kinds/varieties of fruit" although I usually wouldn't bother specifying. The first time I ever heard "fruits" was from my Chinese friend who could only half speak english (when I was about 24), and it was so weird I never forgot it.
Edit: After some internet searching. No one seems to be able to agree. But it seems common enough that it should be accepted. The best explanations I found were here: http://english.stackexchange.com/questions/25147/is-using-fruits-as-the-plural-of-fruit-acceptable
I still think it sounds weird.
If somebody said "I mostly eat fruit for breakfast", I would assume that he/she is eating more than one type of fruit. (Eating just one would be pretty boring!) Likewise, if I wanted to explain that a store sells apples, oranges, etc., I would just say "The store sells fruit." Using the plural would be unnecessary, but it would not scream "bad grammar" to me.
Sometimes the plural is used when you want to emphasize that you are talking about a variety of fruits. "You should eat fruits and vegetables for good health", or "I like all the fruits of the world." And if you want to emphasize that you are only talking about one type of fruit, say so: "I mostly eat one type of fruit for breakfast."
If you are woefully confused, just say "fruit", and you should be OK :)