You may visit Hong Kong Immigration Department website:

http://www.immd.gov.hk/ehtml/hkvisas_4.htm#note%20for%20visa%20req

But, there are contradictory information there for Nepalese passport holders:

Nepal belongs to following catagories:

1. Always require VISA, except in direct transit by air and when the person does not leave the airport transit area

2. All nationals (except holders of Diplomatic and Official passports) are required to hold a valid visa for the HKSAR for whatever purpose (including those who are in transit and remain on the airside).