As mentioned before, if you do 6 and 6 and on your time off return to your home in Canada, you pay tax like any other Canadian working in Canada. However, you have tax a credit available called the OETC (overseas employment tax credit) which lowers your tax rate to approx 15%.
You have to become or be a non-resident of Canada if you want tax free earnings. You will however be subject to tax in your country of residence if applicable.