While I haven't been looking for casual work for quite a while now, anecdotally there is loads of work around. Obviously you need to be willing to work retail/similar though.
And as for where to look that depends on where you are going to live. Assuming you'll be living on campus/in the inner north then:
- Supabarn in civic (a supermarket)
- Woolies in Dickson
- There are hundreds of retailers in the Canberra Centre (civic)
- It's a quick and convenient bus-ride to Belconnen Mall where there is a Coles, Woolies and all the other usual retailers.