From the handbook:
"Students may be eligible to exit the double degree program and graduate with either a Bachelor of Engineering named degree or a Bachelor of Science after 3 or 4 years, depending on the units studied.
Students who wish to graduate with a Bachelor of Engineering degree prior to the completion of the double degree must have completed at least 192 points of studies, including all of the requirements in Part A, B, C and D for the particular Engineering specialisation. Students who wish to graduate with a Bachelor of Science prior to the completion of the double degree must have completed at least 144 points of studies, including all of the requirements in Part A and B for the Bachelor of Science degree."
So you could just satisfy the requirements of your engineering degree and then exit without doing your science degree. This might be the best option in case you have a change of heart in a few years time since it would allow you to still opt for the double degree if you end up changing your mind.