I have been a contractor for about 9 years now (as has DH), we work on the principle that we put about 35% of earnings away to cover tax, gst, acc etc. We also put a lot of our costs through the company (obviously only valid costs that are related)...like internet, cellphones, carparking etc, which means that we can claim the gst on those.
As Joshierocks mentioned - it can be quite time consuming with all the tax paperwork, and it pays to keep on top of it...but you do get to charge a much higher hourly rate.
Being a contractor is different from having a fixed term contract, and there are different considerations when it comes to tax....
We have found that having a good accountant is invaluable, as if you are registered as a company you will need to do yearly returns etc.