These species of agamid lizard are native to China and mainland Southeast Asia...but I understood that they are locally extinct in Hong Kong.
They are the lovely Chinese water dragon (Physignathus cocincinus). They make great pets and perhaps quite a few have escaped or been deliberately let go, as we are seeing more and more of them on our walks. So re-establishing a population is not a bad thing, and they should not upset the local ecology.
Here it is sunning itself on a pipe, halfway up Tai Mo Shan.
It is also known as the Asian water dragon, Thai water dragon, and green water dragon.
oh, and the same one, with a different angle. Actually, I saw two at this location.