By having a look at the following table regarding the Bachelor of Engineering (Honours)/Bachelor of Science (Computer Science) from UNSW's
Admissions Information website:
| ATAR (excluding Adjustment Factors) | Selection Rank (ATAR plus any Adjustment Factors) |
Highest Rank to Receive an offer | 99.95 | 99.95 |
Median Rank to Receive an offer | 94.83 | 99.95 |
Lowest Rank to Receive an offer | 81.1 | 87.9 |
And, by considering UNSW's statement that the selection rank will provide a better indication of the likelihood of receiving an offer to the chosen program, it can be inferred that a minimum goal would be to achieve an ATAR of 90. Subsequently, adjustment factors (gained through HSC Plus for instance) will support your application and should give you a good chance of receiving an offer to study this double degree program. Of course, given the above, the higher the ATAR that you can achieve (and whether you can benefit from any adjustment factors), the higher your chance of receiving an offer. Based on this, I believe that an ATAR of around 95 plus any additional adjustment factors will result in a much higher chance of receiving an offer.
With that being said, keep in mind that this is 2021 data and UNSW is yet to provide 2022 data regarding admission into the programs that it offers.
I hope this helps!