If this is a lump sum housing allowance, then this is all taxable. This is not an reimbursement of rent paid by you.
Provided that the employer has established clear guidelines to control and has exercised proper supervision over the reimbursements of either the whole or a part of the rent paid by the employee as tenant to the landlord, the Assessor will accept such arrangement as if the employer directly providing a place of residence to the employee. Ratable Value (RV ) will be calculated and included in the employee’s Assessable Income and the reimbursements of rent will not be treated as income.
However, if the employer does not control how the employee would spend the money or has not exercised proper control over the expenditures, the Assessor will regard the reimbursements as cash allowance and include the full amount as income in the Assessable Income.
More examples in
http://www.ird.gov.hk/eng/pdf/pam44e.pdf