Casual worker's fee tax deduction

I'm a registered medical technologist currently unemployed but I'm accepting empoyment as a reliever/on call med tech. I join annual physical exams/medical missions and the pay is on a per day basis. I was informed of a certain fee and later on found out that tax would be deducted from it. :(

1. Is it necessary to deduct tax from casual workers?
2. If so, at what percentage?

Need expert opinion since i couldn't find anything on the net.