Most HK banks will not do currency exchange unless you are a customer.
Canadian to HKD or any Asian currency locally is always bad due to low volume. Last week they were buying at $5.6 and selling at $6.5.Base on Toronto money changer rates for CDN to US and HK changer US to HK you do better exchanging for US $100 new notes and then changing it to local currencies in Asia.
There is a circle of Canadian expats and Hong Kongers with Canadian citizenship/relative living in Canada who do person fx rate exchanges.
I usually ATM HKD for incoming Candain visitors in exchange for Candian dollars, cut out the middleman.