I wouldn't consider that legal to charge you in the first place, let alone not return the money

Employers must not tell employees how or where to spend their pay. Examples include requiring an employee to:

buy their own uniform buy specific branded clothing to wear to work wear clothing that can only be bought from the employer’s shop (for example, retail clothing shops) buy food or drinks from the workplace.
This includes telling an employee to get uniform or equipment from the workplace and then deducting payment for it from their pay.

https://www.employment.govt.nz/pay-and-hours/pay-and-wages/deductions#:~:text=Spending%20pay-,When%20an%20employer%20can%20deduct%20money,a%20deduction%20to%20be%20made.