This stream trek is not easy or risk-free even for experienced rescue teams, especially in the rain and/or in the dark. Flash floods kill people out stream trekking most years in HK. In the dark it is doubly risky. There is nothing that really counts as a "trail" anywhere near the stream in question.
The site is only a couple of kilometres from the GFS base, so in the daylight after heavy rain with high risk of flask floods I can see that a helicopter might be preferred. Carrying someone out on foot from some of these places could be pretty tricky, even for trained teams.