To be honest, I'm not sure. I dropped someone off outside and they went inside and did it. I'm sure if you ask the information counter in the building they'll be able to tell you what time the service is available.
thanks, had already done a web search and found the Home Affairs Dept! They did say I could do it at the Centre, Unit 5, G/F, 99, Queen's Rd Central (as well as in other offices in HK).