I was considering doing the same thing with melbourne arts as I thought it would be more interesting. I was interested in the same things as you. I decided not to, but I still think it would still be an awesome thing to do. Theoretically you can definitely do further study in economics if you major in it through arts, and I don't see why you couldn't also in practicality. I'm no expert though so you should probably call up the university and ask them. I'm 95% sure this is considered a very common avenue into study of economics at Masters/PhD level, as if you look at famous economists with PhD's, many of them started through arts. Also, if you consider the US system, there is no 'commerce' degree and as a result students who commence further study in economics would almost always have a tertiary background which looks very different from someone with an australian 'commerce degree'.
Studying economics through arts also allows for development of a unique perspective on the area (as opposed to the commerce perspective) as you are able to study Economics/Philosophy or Economics/Literature or Economics/Politics which all complement each other very interestingly.
I decided to study economics through commerce as I was given the opportunity to do so, and I am more interested in doing a MBA. Of course, I could still do an MBA with an Arts background, But I want to work in Banking for the 2 year work experience requirement, and with australia's bizzare, vocational, tertiary system I am forced to play the game and do commerce to maximise my opportunities in working in the merchant banks.
Hope this helps.