It'd be utterly ridiculous to call plagiarism for words - we all use the same words, more or less, it's the way we put them together to express our ideas that's unique.
Phrases - a phrase or clause would seem fine to me; full sentences are tipping it a bit for me, but maybe okay. (In probability terms, the more words in the chunk, the less likely it is that multiple people would come up with something very similar.) But English is all about mutually learning from each other, give-and-take; that grade-1-spelling-test attitude that says 'mummy he copied myyyy phrase wahahahah' is ridiculous. Totally up to your teacher really what they do about it in SACs, so ask them - I doubt VCAA policy would come into play.
Paragraph - now that
is plagiarism; you're copying their ideas in the exact order and way that they wrote them. Nothing wrong with taking something very short and adapting it into your own ideas and writing, but the bigger the chunk, the less it is
your adaptation of it and the more it is just their work. The point of English is developing your own opinion/ideas/stance, so it's just not okay to copy. They should at least cross out the stolen paragraph when marking.
But will you get caught in the exam? Probably not.
But examiners aren't idiots. My sister (proofreads scientific journals) catches plariarisers because of stylistic differences. Examiners will recognise any weird disjointed style as it's almost impossible to adapt stolen phrases/sentences/paragraphs appropriately to your own style. In other words, if you want to cheat, develop some skill and finesse.

If you get caught in an exam (it'd have to be more than phrases, more like paragraphs), I imagine you risk a big fat 0 for the essay.