In March 1949 the government committed to building at Tamblyn's Orchard and plans were prepared to enable work on the diversion channel to start in June 1949.

Many of the design decisions were based upon results from studies undertaken from 1949 to 1954 on a 1:80 scale model of the dam at the Department of Scientific and Industrial Research's Hydraulics Laboratory at Gracefield, Lower Hutt.

Therefore, he put in place plans to build a construction village and in July 1949 had the MOW commence work on the excavation of the diversion channel.

Eventually the village grew to 724 houses complete with a 90-bed hostel, a 600-child primary school, a cinema, a social hall, 17 shops, three churches, a fire brigade and ambulance building, four tennis courts, a swimming pool and a piped sewage scheme.

In May 1946 the PWD and the New Zealand Railways Department held a meeting to determine what would be needed to transport materials to the proposed site of the power station.

[11] As well, four tunnels[1] including those at Manuka and Round Hill on the line restricted the physical size of what could be transported, leading to consideration being given to enlarging them.

Consideration was given to extending the line to the construction site but this wasn't proceeded with due to the difficulty of getting past the east end of the existing Roxburgh suspension bridge.

At the construction site a 220 ft (67 m) long single-lane Bailey bridge with a carrying capacity of 24 tons (24.4 tonnes) was installed in 1949 to provide access across the river.

The Ministry of Works had identified that it had a shortage of the engineering and drafting staff to undertake the large amount of power station construction that the government had committed to in the North and South islands.

The State Hydro-Electric Department didn't want to be restricted to one electrical equipment manufacturer and also saw the offer as a threat to their transmission line construction staff.

Taking these concerns into account and wishing to avoid using up precious overseas funds the offer was formally rejected in September 1949 by the Minister of Finance in the Labour government.

After negotiations with Hannen, Holland & Cubitts of England who were joined by S A Conrad Zschokke a revised bid was received and on that basis a contract with a target estimate of £8,289,148 and a 4% fee of £331,566 was awarded on 25 July 1952.

The consortium bought from overseas 82 engineers, supervisors and administration staff and 322 workmen to the project and took over the civil aspects from the Ministry of Works on 29 Sept 1952.

Zschokke who had expertise in the construction of hydraulic structures were limited to only providing engineering services while Cubitts personnel filled all the management roles.

By March 1953 the Ministry of Works became concerned at the progress being made by the consortium and that their management team lacked the experience to construct a hydro power station, which was highlighted by the large amount of rework being undertaken.

Progress was not helped by the Government directing the employment of a large number of assisted immigrants many of whom had little construction experience and limited English.

Labour relations were also deteriorating due to uncertainty over the management changes, reduction in working hours to 40 per week and the impact of cost overruns on the workers pay.

With it having been necessary in 1953[16] to introduce power rationing in the South Island due to a shortage of generation the government decided the slow progress couldn't continue and requested two directors of Downer & Co, a major New Zealand construction company to attend in two days time a meeting at the Prime Minister's summer cottage on 24 April 1954.

The preliminary works for the diversion of the river got off to a bad start when the explosive charge used in mid-June to remove the upstream dumpling damaged the steel sheet pile cofferdam downstream of it.

A 20 ft (6.1 m) deep low pressure consolidation grout curtain was installed on the upstream side of the dame and extending into both abutments to improve the strength of the rock under the dam and prevent leaks.

Fletcher Holdings subsidiary, Stevenson & Cook manufactured and installed the penstocks, the steel frame of the powerhouse and the spillway gate winches The rolled plates for the penstocks were transported by truck from their factory in Port Chalmers to site where a workforce of 80 men fabricated the plates using automatic submerged arc welders into sections in a purpose built workshop and then installed them in position.

Fortunately sufficient time became available to re-make all the joints when from 24 November 1955 for 23 work days up until the Christmas break the members of the New Zealand Workers Union were on strike in support of a union crane driver who had refused to lower a load being carried by his crane when the siren went for a tea break which the contractors estimated would delay the commencement of lake filling by two months.

[18] To connect the new power station to the major load centres, a 52 mi (83.69 km) long new 110 kV wood pole line was first built to Gore.

The linemen then commenced constructing an 89 mi (143.23 km) long double-circuit 110 kV overhead transmission line using lattice steel towers to the Halfway Bush substation at Dunedin which was completed in July 1955 at a cost of approximately £500,000.

The principal connection, however, was a new 266-mile-long (428 km) 220 kV single-circuit overhead transmission line built using lattice steel towers from Roxburgh to a new substation at Islington on the outskirts of Christchurch.

Meanwhile at 12:30 pm on 23 July a speedboat driven by Ken Harliwich and accompanied by Willis Wetherall to leave Roxburgh for Alexandria making the first boat trip on the new lake.

The limited time taken to commission the generating units had meant that no heat runs were performed, which would have identified the impact of this modification on the stator winding temperatures.

At the base of the spillway were three 80 ton (81.3 tonne) low-level sluice gates supplied by Stahlbau of Reinhausen in Germany designed to pass 2,300 m3/s (80,000 cu ft/s).

Two 118 ton (120 tonne) overhead cranes manufactured by Sir William Arrol & Co run over the full length of the powerhouse, including the unloading bay.

Floods in 1979, 1987, 1994 and 1995 have led many residents of Alexandra to put pressure on the owners of Roxburgh power station to better manage the sediment build-up.