Do you have internet banking? Very easy, you can check with any bank.
買外幣不要手續費, HOWEVER, there is a spread on buy and sell rate.
E.g. NZD
Bank Buy rate is 4.64 (i.e. buy your NZD), but Bank sell rate is 4.68 (i.e. sell NZD to you).
For every HKD1000, there will be a HKD$40 difference.