The project would allow the necessary long-term replacement of an ageing 7km section of the existing western interceptor nearest to the Mangere treatment plant.
About 1.26 million urban residents are currently connected to Watercare. The council predicted this number would increase to 1.65 million by 2031.
However, the western interceptor would have to be replaced in as little as 15 years.
Mr Ford said it was effective to extend its replacement to Western Springs to allow for population growth and make it big enough to enable a substantial reduction in wet-weather overflows from the combined sewer-stormwater system of that part of the city.
Watercare had fully examined alternatives put forward by critics.
Alternatives included local storage tanks or local treatment systems, or separate pipes for sewage and stormwater like most of Auckland has.
Mr Ford said the central interceptor project would save the region at least $500 million and give a better result.
The Watercare evidence promises 80 per cent reductions in raw sewage and polluted stormwater, which spills into Waitemata Harbour streams just about every time it rains.
The Mangere Bridge Residents & Ratepayers Association says the project goes against best practice for the treatment of wastewater and stormwater.
Its says the tunnel is a "fix" that is neither quick, economical nor ecologically sound. The association says it is not good ecological practice to transfer large amounts of water from its natural catchments to a shallow enclosed harbour.
The Manukau Harbour Restoration Society argues the project will be improving the water quality of the Waitemata without improving quality on the Manukau, where Watercare has consents to discharge sewage during storm events and equipment failure.
The Laingholm District Citizens Association is concerned with the amount of stormwater in the main tunnel.
Watercare said the central interceptor would not result in the annual daily average flow to the Mangere plant exceeding the permitted discharge.
As a result of the $500 million upgrade of the plant, the condition of the Manukau had improved substantially and was no worse than that of the central Waitemata Harbour and was better than that of the Upper Waitemata Harbour and Tamaki Estuary.
The St Lukes Environmental Society was concerned that Meola Creek would bear the brunt of future overflows and be ruined.
Project designer Clinton Cantrell den-ied this, saying the project would reduce volume of overflows into Meola Creek by 80 per cent for a year of typical rain for 50 years. It would benefit the stream habitat by reducing high peak storm flows.
What it'll cost
$800m The cost of the proposed central interceptor plan
13km Storage tunnel and 6km of linking sewers for the western area of the Auckland isthmus.
$500m What the interceptor project would save the region.
80 per cent Reductions in raw sewage and polluted stormwater which spills into Waitemata Harbour streams just about every time it rains.
* Watercare's full evidence is on www. watercare.co.nz
