A rat snake
There is a very large Common Rat Snake that I see occasionally in Lions Nature Education Centre/Park in Sai Kung. I was there the other day with the wifeywife, and we stood back and watched as he hunted around the dragonfly pond, going into the drainage holes to try and find his lunch.
Common Rat Snakes are found all over Hong Kong including Hong Kong Island. They eat rodents, lizards and occasionally birds and as such can be found from forested areas to areas around lakes and rivers as well as water culverts and jogging trails. Active during the day they are frequently encountered in Hong Kong.
For more information on snakes and ID please do see our sister site: www.hongkongsnakeid.com