Your subject combination seems appropriate and consistent with your future study interest. You will be taking the all-new Software Engineering as well as Design and Technology, which should support you in gaining a solid foundation of knowledge prior to commencing tertiary studies, and can therefore make the start of your next education journey easier (once you reach that stage). Business Studies provides you with a good understanding of the business world as well as four key business functions, which is never bad to have. Mathematics Advanced provides you with an appropriate level of knowledge of mathematical concepts. However, my understanding is that Mathematics Extension 1 is considered important if you wish to study software engineering beyond the HSC, and may also be treated as assumed knowledge (as is the case of UNSW), meaning that you may wish to consider it.
If you choose to take Mathematics Extension 1, you will still need an additional subject to meet the required number of units. In such a case, you can either choose to take a 1-unit subject (see Average Boreduser's suggestion above), or take a 2-unit subject (resulting in a total of 13 units), specifically the all-new Enterprise Computing, which replaces Information Processes and Technology. Given your interest in studying a technology-related field, this subject can potentially further support you in gaining a solid foundation prior to attending university, whereas another unrelated subject would not. If you choose to take this subject, you will have the required 12 units anyway, meaning that you do not necessarily have to take Mathematics Extension 1 if you do not wish.
With that being said, it is important to keep in mind that, in choosing your subjects, you should prioritise your personal interests as well as your performance (i.e. subjects that you believe you can perform well in).
I hope this helps!