The man-made reservoir has a size of around 1.8 km2 (0.69 sq mi)[3] and extends over its northern arm in a 45-degree left curve over a length of around 3.1 km2 (1.2 sq mi) and over its eastern arm to the southern tip over a length of around 2.8 km (1.7 mi).

Its widest point is in the middle part of the lake and extends over a length of around 900 m in a north-west-south-east direction.

[2] Lake Otamangakau, which is located at an altitude of 611 m, is fed by a few smaller streams from the surrounding area and the lake experiences its regular outflow via a small stream that flows into the Whanganui River around 1.7 km (1.1 mi) further.

It has a length of around 300 m with a crown width of around 12 m, as Forestry Road runs over the dam.

Directly towards the lake, the barrier measures around 114 m at sea level and around 60 m on the western side facing away from the lake.The water outlet in the direction of the Whanganui River is on the northern side of the structure.