I think you should clarify it with the UNSW faculty of Engineering. The best way is to talk to academic planner who are available for all faculties including engineering.
I saved you the hassle of finding out directly linking you to the site.
http://www.eng.unsw.edu.au/info-about/contact-us
You can email but i found that emailing takes alooot of time. so best contact them on student enquiries at the engineering school offices. Click on Student enquiries.
I had the same problem as you. When i got into Commerce and law it said 60 months course when on the outline it clearly said 5 years. So i called them up. Apparently you might have applied to a DIPLOMA of Engineering which is like a side off of an actual Bachelor of engineering course.
So a bachelor of engineering is 4 years gives you the proper degree while a DIPLOMA of engineering(48 month) at UNSw gives you only a limited qualifications of an engineer. Better confirm it with UAC as well. They might have mistaken your UAC application as well.