Normally no service charges are required for exchange of foreign currency. Banks earn from the buy-sell spread. For example,
Australian Dollar: Bank Buy @ 6.8615 and Bank Sell @6.9045
That means if you want to buy A$10,000 from the Bank, the Bank will charge you HK$69,405.
On the other hand, if you sell the bank $A10,000, the Bank will only pay you HK$68,615.
Different banks have different exchange rates and buy-sell spreads. If you buy/sell a lot, you may need to compare a few banks.
Btw, banks usually have service charges if you withdraw foreign exchange notes over certain limits. And TT rate is usually better than banknote rate. You may ask the bank for details.