From Tauplitz drive to the south over road 145 until you reach Lessern. Take the exit to Lessern and try to park your car somewhere near the road. Parking is a problem and when I visited the Lessern wasserfall July 2020 I parked the car near a house in the grass, but this wasn’t ideal. When I walked upstream the river there was an unofficial parking spot at the beginning of the trail, when going under the main road (gps 47.547596, 14.039172).
From Lessern a trail leads upstream the river Mühlbach to the Lessern wasserfall which can be reached in 15 minutes. The trail is quite easy but some parts go up, especially at the end. The trail runs along the river in a small gorge, very beautiful to see. Although it is a short walk it is a nice walk with children but be careful at the end.The Lessern wasserfall drops down in a single drop of 32 meters.
The afternoon is the best time to visit the Lessern wasserfall (Mühlbach wasserfall) and I think late spring and early summer the flow of the river Mühlbach reach its maximum capacity.
A little further in the direction of Schladming you will find another, even more picturesque, waterfall with a height of 23 meter at Salza, called the Salza wasserfall.