The new dam was long at the top, high with a bottom thickness of and a top thickness of widened by cornices to 2ft 6in (760 mm), to form a footway, which extends over the by-wash in two 13 ft 6 in (4.1 m) spans. This footbridge and the scour-valve platform were the only portions of the dam in which steel reinforcement was installed. The spillway was widened to . No great difficulties were met during construction, except that it was necessary dig to lower than originally intended in order to find a hard, and watertight bottom which was found to extend for in length from the north end to the centre of the stream. Beyond this to the south end of the wall the excavation was taken down to a depth of from to lower than originally intended. A cast-iron gate valve was installed to allow emptying and letting sand out of the lake. By February 1918 the dam had finished and refilled with water. With the demand for electricity within the Borough of New Plymouth showing no sign of levelling off, and with applications for power been received from other local bodies, the council committed to expanding the Mangorei scheme. The council in late 1918 engaged consulting engineer John Blair Mason (1858 to 1927) of Blair Mason, Lee, Owen and Cree Brown, to investigate the various options. Mason offered a number of options but Scheme D which also had a partial option called E11 became the preferred choice of the council. Scheme D called for tapping the Waiwhakaiho River at a new upstream intake and conveying the water via a tunnel to a lake created by a new high dam on the Mangamahoe Stream about 30 chains (603 m) above the existing dam. This high dam would give a head of above the powerhouse, submerge at least 30 acres (12 hectares) and with a draw off would provide 13,000,000 cubic feet (368,000 cubic metres) of storage. The lake would rise above the saddle about half a mile (800 m) south of a forebay from whence it would be conveyed by water races and penstocks to the powerhouse. The actual storage required with a load factor of 0.45 to develop the maximum water power would amount to 7¼ million cubic ft (205,000 cubic metres). Mason estimated that this would give a maximum of 6,300 hp. His estimated cost for the project was £46,000. Scheme E11 on the other hand he estimated would cost £11,700, of which only £1,300 need be regarded as temporary work, being the estimated cost of race and depreciation on pipe line.