The 480m long diversion tunnel was constructed by blasting through solid granite using the controlled drilling and blasting method. This was done at the start of the construction to divert the upstream stretch of the Sungai Selangor River before the construction of the main dam. The tunnel is 7m diameter and of horseshoe shape.

The long-term use of the diversion tunnel is to house the twin 2200mm diameter discharge pipes coming out of the draw-off tower. The water drawn from the dam via the draw-off tower flows through these twin discharge pipes for downstream release into the Sungai Selangor River.