The Copyright Act provides the owners of copyright material with a suite of economic rights and creators with moral rights. However, in most cases these rights only apply to the extent that someone uses a substantial part.
So what is a substantial part? The answer is complex and depends upon the circumstances, as there is a large amount of case law on the subject.
As a very broad generalization, it is safe to say that if you have used any part that is recognizable and distinctive to the average person, regardless of how much you copied, then you will have copied a substantial part. The same applies if you have used an important part. Consequently, there is no fixed percentage of a work that can be claimed to be an insubstantial part, as the quality of what is copied is more relevant than the quantity. Also, you cannot get around copying a substantial part by simply changing bits of the copied work.
This should not be a concern, however, as there are adequate exceptions that allow staff and students to copy material without having to rely on the substantial part provisions as a defence.
Please note: Part VB of the Copyright Act (the teaching provisions) refers to copying insubstantial parts. This is an unfortunate and confusing choice of terms, as it does not define the threshold of what constitutes a substantial part, but rather only states what may be copied in reliance upon the statutory licence (Part VB) without having to pay remuneration to the copyright owners. Staff do not need to worry about whether the part they copy is remunerable or not, as we operate under a sampling licence agreement, and it is the Copyright Agency Limiteds responsibility to sort out what is an insubstantial part from the information they collect during the sampling period.