Voluntary services are offered by choice - it is not mandatory or coerced. It contributes to the well-being of an individual or the community, and is usually coordinated by a non-profit or public sector organization, and does not pay any salary or wage (although you may have your expenses covered).
Voluntary work is often equated with unpaid work, and people who contribute their time and energy freely to build a better community don't necessarily get the support systems and recognition they deserve. However regarding people working as volunteers during their summer holiday, they will get accommodation and often a small amount of money for their personal care.
Most volunteer work in Hong Kong requires that you speak Chinese as well as English but nevertheless you can find some website that might help
you find something like Ho Sum:
www.ho-sum.org/eng/index.jsp
You also have the government advertises some volunteer work here!
Of course you also have some obvious NGOs and volunteer companies like Oxfam Hong Kong, Rotary Clubs, Lions International and not so obvious ones like Friends of the Earth Hong Kong (www.foe.org.hk/index_enhk.html), American’s Women Association (www.awa.org.hk) or also the Changing Young Lives Foundation (www.changingyounglives.org.hk). You also have the Red Cross in Hong Kong (www.redcross.org.hk), WWF in Hong Kong (www.wwf.org.hk/eng/index.php).
You can also contact you home consulate to see if they have addresses or if they look themselves for someone.