In a November 26, 2004 External Technical Interpretation (2004-0101151E5, Hewlett, Randy), CRA notes that there is no taxable employment benefit when an employer provides free parking if the employee is regularly required to use the vehicle to carry out the duties of employment. For example, if the employee is required to use the vehicle at least three days a week for employment-related travel, the employee would not be in receipt of a taxable benefit. Otherwise, a pro-rated portion would be a taxable benefit.
For further information see VTN Monthly Tax Update Seminar, Issue No. 282.