The Materuni Waterfall is accessible by a footpath from the hiking start point. There are multiple “guides” offering their services to bring you there. They also take an entrance fee to the area, even if there is no real governmental park sign or something. In our case we paid 10.000 Shillings for the entrance fee and 10.000 for the guide per person. If you do the coffee and lunch or need a lift from or to Moshi they will charge you extra.
From the hike starting point it is a ~40min walk through the village and forest before you arrive at the waterfall. It’s a beautiful spot in the middle of nature. You can also go into the water at your own risk. We did a refreshing dip in the pool there.