From the best of my knowledge, an academic, a translator or tour guide. There may be more, but I don't know. You could also work for ASIO and ASIS if DFAT doesn't work out, but I wouldn't recommend working in such areas. Have a read of these organisations, and decide for yourself though. The ADF would be better, but if you do choose to join the ADF, since you're female, JOIN AS AN OFFICER. As you may have heard from the news, being a female in the other ranks of the ADF can be quite dangerous.
http://www.asio.gov.au/
http://www.asis.gov.au/careers/Overview.html
Two degrees aren't better than one if one of the degrees won't be used!
It won't be mostly science. The number of units you have to do to earn your Bachelor of Science would/should be the same as the number of units to earn a Bachelor of Arts in a double degree programme. My advice is: if you don't like science then don't do it, really. I understand that you want to be versatile, but you should really be doing the subjects that you enjoy studying or else you may fail some of the courses that you are doing, which means a longer stay at uni.
To work for Australia as a diplomat at the UN or at an embassy, you would have to go through DFAT.
http://jobguide.thegoodguides.com.au/occupation/Foreign-Affairs-and-Trade-Officer
To be a translator at UN read the following-
https://careers.un.org/lbw/home.aspx?viewtype=LCEFD&FId=7
I don't know about NGOs.