How to apply for Hong Kong Visa for Family and charges involved?

My family that includes Myself (Father), my wife, Son (8 years), daughter (4 years) basically two adults and two children are planning to be in Hong Kong from January 20th to 24th for sightseeing and holiday. We all have individual Sri Lankan passports however residing and working in Melbourne/Australia legally as Permanent Residents. We have already booked the flights to and from Hong Kong and hotel for the stay. As Sri Lankan passport holders we understand we still require to apply for tourist type one. What would be,
1. If Visa is required, Can you let us know how to get Hong kong one from Melbourne or would it be also possible to get it from Colombo?
2. Considering we all have individual passports including the children should we apply on 4 individual application forms?
3. What would be the fee?
4. How long would be the process?