Hi Cam,
I am doing Science (Advanced)/Arts but I think I know enough about the Environmental Management degree to help you out here. Sounds like you've done a bit of research and you're definitely on your way to making a good decision. The Environmental Management degree is more specialised than a general Science degree and is definitely a good choice in terms of consultancy and environmental departments in corporations e.g. banks, extractive industry, and government. It incorporates mainly science courses but a few "management" courses such as economics, managing people, managing natural resources, and a law unit - but don't let these scare you by any means, they are all designed for the environmental management program.
Your Arts degree could help you potentially get a job in a global organisation as a second language (Spanish or French usually required) is a must have for the UN etc. Utilise the uni open day (September 5th) and even visit the student science centre, or send an e-mail. Generally being a good communicator will help you in any career as well - you just have to weigh up how long you want to be studying for.
Remember that when undertaking a double degree, if you should decide to pursue only environmental management or arts (or something else entirely) you can do so quite easily within the first 1-2 years without losing time as a single degree allows you to have a reasonable amount of electives.
In the meantime I would suggest you focus on your HSC and maybe before the open day think about what major in environmental management you may enjoy most.
Best of luck!