Water first flowed through the tunnel on 24 Dec 1926. The tunnel is described in item 164.g of that year's PWD Annual Report:
This Tunnel was “holed” on 9th. August or about 16 months after driving was commenced at a point 149 ft. from the North Portal.
It was found economical to extend the ‘open cut’ section at the North end for a distance of about 140 ft. further than was originally intended.
This resulted in reducing the length actually tunnelled to 2,020 ft. The portable compressor engaged on the open cut work also drove from the north end the 149 ft. of Tunnel referred to above, the remaining distance of 1,871 ft. having been driven from the central depot between the North and South Tunnels.
The rock throughout this Tunnel was found to be generally of a poor quality, numerous patches of shale, soft clay and in two cases running sand and clay were encountered.
Altogether eleven separate lengths of lining were required having a total length of 362 ft.
Some difficulty was experienced in depositing the lining owing to seepage from the soft patches already referred to.
The lining is of 6 to 1 cement concrete of minimum thickness of 6" and 12" but in one length traversing a seam of loose clay and running sand cement concrete packing was put in to a height of 20 ft. above the normal level of the Tunnel roof.
The minimum Tunnel dimensions are as follows: —
Width at invert 7' 0"
Width 4 ft. above invert- 9' 0"
Total height 7' 6"
The roof is arched to a radius of 4.65 ft. and the invert is dished to a radius of 12.50 ft.
The tunnel has a total fall of 9". or 1 in 2,700. and the invert has been graded smooth with a layer of cement concrete of a minimum thickness of 3".