Auckland plans to spend $4.8 billion over the next 10 years on water projects as the city's population grows.
The biggest project for water retailer Watercare Services - controlled by Auckland Council - is an $800 million waste water tunnel which will go from Western Springs to the Mangere Waste Water Treatment Plant.
It will replace an aged system and reduce overflows from combined sewerage and stormwater pipes, which contaminate the beaches and streams of central Auckland.
Auckland Council population predictions, the Unitary Plan and proposed developments guide the capital spending programme.
Auckland is expected to have one million extra residents by 2040.
Watercare's chief infrastructure officer, Graham Wood, said 41 per cent of the spending was to meet regional growth, with the rest split between renewal of the $7.8 billion of assets already in use and meeting the required service levels for public health and the environment.
He said Watercare took the medium of the council's population predictions and used these to plan projects such as the $136 million upgrade for the Mangere plant and the $48 million Waikato River water treatment plant expansion.
However, it had to put assets in the ground years ahead of development around the Massey North town centre and Kumeu, Huapai and Riverhead.
Watercare was working with the council on its economic growth such as the Southern Initiative and in the region's north and west, and felt it had the infrastructure in the ground to cater for the next three to five years of development.
The company supplies 1.4 million customers and expects to earn $496 million this year and borrow $1.27 billion for capital projects.
It is required to be a minimum cost provider and cannot pay the council dividends like its other investments.
Mr Wood said it paid for projects through an infrastructure growth levy on developers.
"We try not to get existing customers to subsidise developers. But existing customers pay for upgrading through their water and wastewater bills."
He said some of the projects had 100-year asset lives, which allowed the company to borrow against its revenue and collect the money over time.
Watercare was able to get loan money at a reasonable rate because its loans were guaranteed by Auckland Council's excellent credit rating.
It spent $330 million a year on projects, of which $150 million was going to clean up urgent problems with rural services, such as the Pukekohe water supply and Helensville sewage treatment.
Pukekohe's new water supply - once notorious for an "orange tint" - was being opened in September and its new connection to the Waikato River plant at Tuakau meant it had no problems with meeting its growth plans, Mr Wood said.
Since the Waikato River plant was built in 2002, out of security of supply fears, it had been expanded to provide from 75 million litres a day to 125 million litres in December.
Work had started to boost production to 150 million litres a day.
Auckland needs 370 million litres and during the drought the plant was able to provide 27 per cent of demand.
Watercare's assets include 16,500km of pipes, reservoirs, dams at Hunua and Waitakere, treatment plants at Tuakau, Ardmore and Huia and sewage treatment plants at Mangere, Rosedale and Army Bay.
1. Waikato River water treatment plant expansion
Cost: $48 million.
The three-year project increases capacity of the Tuakau plant which was built in 2002 to treat Waikato River water for sending through a 37km pipeline for Auckland use.
The project increases capacity of the membrane filtration plant from 75 million litres a day to 125 million litres a day. Work has started to boost that to 150 million litres a day. The original plant and pipeline cost $100 million.
2. Hunua No 4 new water main
Cost: $350 million.
This main is 1.5m to 1.9m in diameter and will carry treated water 28km from the Redoubt Rd reservoir in Manukau to Campbell Cres in Epsom. Along the way, it will connect with supply networks in urban Manukau and Auckland. Eventually, the big main will extend to the reservoir in Khyber Pass Rd, Grafton. Construction started in May last year in Manukau Heights and runs through to 2017. For most of the route, the pipe will be buried under roading by open trenching but a tunnel boring machine is used for getting under busy roads. The pipeline also crosses railway lines and Manukau Harbour.
3. North Harbour water main duplication
Cost: $240 million.
The pipeline of 900mm to 1200mm diameter will carry water from Waitakere Ranges dams for 32km from Titirangi to Albany to supply North Shore communities. A new booster pump station will be built at Schnapper Rock. Stage one is due to finish in 2017 and stage two will carry on to 2026.
4. Central Interceptor tunnel
Cost: $800 million.
A tunnel-boring machine will be used to make the 13.3km of 4.5m diameter conveyance and storage pipeline. It will pass under urban Auckland from Western Springs to the Mangere Wastewater Treatment Plant and connect with local networks on the way. The aim is to collect 80 per cent of the diluted sewage which overflows into waterways from combined wastewater/stormwater pipes. Three years' work on the tunnel design has preceded hearings for the resource consents, which could be heard this year. The project is not expected to be finished until 2027.
5. Northern Interceptor
Cost: $270 million.
This will take northwest Auckland's waste water from Hobsonville 18km across the upper Waitemata Harbour to the North Shore's treatment station in Rosedale Rd. Project involves twin 2.8km 800mm diameter rising mains in a trench and 6.2km sewer in a tunnel bored by a remote-controlled machine. Estimated completion is 2020.
6. Mangere Waste Water Treatment Station upgrades
Cost: $136 million.
The plant was upgraded in 2003 in a $450 million project which included removal of 500ha of oxidation ponds from the shore of Manukau Harbour. As the city grows, the new focus is building the plant's capacity to remove biological nutrient from waste in the second stage of the treatment process in order to keep harbour discharges within consent requirements. The upgrade finishes in 2017. Tank-like anaerobic digesters play a key part in breaking down solids and producing gas that helps power the plant's machinery. The No 8 digester is being upgraded at a cost of $15.8 million.