What other say:
Mermaid pool has a total of 2 trails. One is 2 hr walk and another more complex 6hr complex walk through bushwalk. Walk is scenic and the pool has got some good views too. Make sure you very plenty of water as it is not available there. Also, it is not a very well laid path but need to go through the bushes. There are markings laid out in yellow paint and make sure you follow them else there are good chances of getting lost in the way.
The parking area is only a rough dirt track. The whole area is crown land, not national park. The tracks and guides are maintained by volunteers who have done a stellar job in making and marking the trail. In places you need to discern carefully which way the trail goes. The track to the Mermaid Pools has two main options, and both are easy to follow (Grade 3). The track through the Tahmoor Gorge (Grade 4/5) is less clear and you need to be on the look out for the ribbons and paint that will be your guides. Well worth the effort. Total walk 10.5 - 12km depending on options.