It doesn’t really matter where you buy. Your Local Bike Shop (LBS) or somewhere near your work is fine, because you’ll be able to take advantage of their servicing and advice.
Some people will be inclined to shop at shops that stock certain brands, or because they’ve seen shop kit on the road, but honestly it does not matter.
Your key is to get something, and get moving, without prevaricating on the multitude of choices (I discuss this further in What To Buy).
I personally recommend Total Rush, Hampton Bicycles or The Freedom Machine based on my experience. There are loads of great shops in Melbourne.
The key is not getting ripped off, and we’ll discuss that in What To Buy.