Nevermind is actually one word, not two words. But often it is misspelled, so it is ok, people still know what you mean. Just like "Alright" is one word, not two.
Anyway, Nevermind means "just forget about it", so if someone says it to you abruptly, yes, that means that they do not want to hear from you again.
But usually, it is polite, and is another way of saying " no worries , mate" or "sweet as, mate.
It depends on the tone of voice. But us kiwis would never be rude about it!