Useful summary on how and when an employer can deduct from an employee’s wages:

http://www.heskethhenry.co.nz/Articles/x_post/can-you-deduct-from-an-employees-wage-its-not-as-easy-as-you-may-think-00007.html#page=1

Note the common mistake of including a deduction clause in the employment contract. People tend to forget that such consent can be withdrawn any time!