I agree with Mr. M.
GoodmanIf someone says "I bought a 50" LCD TV yesterday from Bestbuy which was scheduled to be delivered today but I haven't heard from the driver yet". By your definition, does that mean this sentence is also a mess?
Yes, in my opinion, but it's a slightly different kind of mess. In this sentence, in the way that you intend it, "from Bestbuy" could be removed and you would still have a complete sentence. If you intend the basic sentence to be "I bought a TV yesterday which was scheduled to be delivered today," then I think "from Bestbuy" should be put in parentheses or between commas. Otherwise the "which..." should actually modify Bestbuy, as in "I bought a TV yesterday from Bestbuy which has the lowest prices."
Goodman "Our milk powder comes from New Zealand WHICH is guaranteed to contain no melamine ..
The structure of this sentence is different. (You can't remove "from New Zealand" and still have a complete sentence.) I'm afraid I can't label all the parts of speech and quote the appropriate grammar rule for you, but certainly if I read a sentence which began "Our milk powder comes from New Zealand which..." I would expect the following information to apply to New Zealand, not to the milk poweder. "Our milk powder comes from New Zealand which is somewhere near Australia."
I'm not saying that no one would come up with a sentence like yours, or that it's incomprehensible. It would not be uncommon to hear or read a sentence like that from native speakers, but it would still be wrong. I'm not sure what the technical term is for what's wrong with it -- maybe "misplaced modifier."