Thanks Lolsmith.
Looks as though Economics is available as a major in Arts, and Computing as a minor in Science, which is fine by me.
What concerns me, however, is that they are not listed as 'specialisations' on the actual Science/Arts combined degree page:
http://www.handbook.unsw.edu.au/undergraduate/programs/2011/3930.html
And I shall tell you why.
If you read the rules of the combined degrees, you shall see.
Basically, if A wants to do a combined degree for eg Arts/Law, they cannot do an economics major because the arts faculty requires a certain amount of UOC (subjects) from them. Typically a major in the arts faculty is required.
For a major in economics, A would need to do economics/law or commerce/law as a double degree means that because A is doing the degrees together, they take less time so something gives.
What gives is the sheer amount of free electives A has from other faculties.
A must do an arts major (perhaps a language or something else rivetting for A).
My recommendation for you if you wish to study computer science and economics is either to do commerce (liberal) or science with a comp sci major and an economics minor. Perhaps comp sci degree with economics minor as apparently comp sci has a lot of elective options.
I'm sorry but I don't think given the current rules that you can study them together in a combined degree because they're out of the home schools