Relative clauses: 多い人 vs 多くの人

Question

I was practising a grammar pattern, so in my personal example I wrote:

多いオーストラリア人

However, a native speaker corrected me with:

多くのオーストラリア人

I understand that in relative clauses that adjectives go before the noun (e.g. おいしい牛乳). How come they corrected me like this? How does this work? Or did I learn relative clauses wrong?

Answer 1

多い is just a special case word, where, on its own modifying a noun, it takes the form 多くの. However, it can take the 多い form attributively as part of a longer relative clause such as オーストラリア人の多い場所 'a place where there are many Australians'.