I've brought in lots of different types of goods by courier and by post.
With post, if it's small enough, you can be lucky. Generally, anything big gets examined, and if it's small but expensive there is a good chance you will be caught.
The problem is that if you are caught by customs, the process of assessing and paying the duty is so complicated that it is just not worth the time and effort to do it yourself. The first time I tried to get something through customs, I had to take a day off work, drive out to Granville (far west of Sydney, not a place you want to spend a day) and it took about 5 hours before they released my package, and that was only after a certain amount of begging. If it's something inexpensive, the process is a little easier, but still time-consuming, and depends on the goodwill of the customs officer you are dealing with.
The other problem with the post is that you cannot track your package, and often it is uninsured.
Couriers offer tracking as well as insurance, fast track through customs and quick delivery. After some bad experiences with the post (things getting lost, delayed for months, packages arriving bent and soaking wet, and torturous customs processes), I am now quite happy to pay extra for a courier company to do it all, including the customs and GST. It costs extra, but to me it is worth it.
Ross