There does not seem to be an ATAR requirement for UNSW's Undergraduate Certificate in Engineering (Bachelor Pathway). This would make sense as essentially, the purpose of this certificate is to give students who do achieve a sufficiently high ATAR for standard entry an introduction to their degree of choice (in your case the Bachelor of Engineering (Honours)). It consists of four introductory courses over two terms.
In response to
@Nedom's question, a student's decision in such a case is likely to be highly subjective. While gaining direct entry into a Bachelor's degree at a different university is obviously better than having to study an undergraduate certificate and then transfer to the equivalent Bachelor's degree, some students may insist on studying at a specific university (in this case, UNSW) even if there will be a need to first complete an undergraduate certificate. Of course, there is an exception in the event that the student's ATAR is not sufficiently high for any of the other institutions, in which case the undergraduate certificate could become the only option. If the specific institution is not a priority, then studying a Bachelor's degree objectively becomes the superior choice.
Additionally, an important thing to note is that enrolling in UNSW's Bachelor of Engineering (Honours) after completing the undergraduate certificate still requires students to achieve an average of 65, putting them in a similar position, should their intention otherwise be to transfer to UNSW from another university.
I hope this helps!