An exploding population and an increase in mostly residential development in Mangawhai means the town's wastewater treatment plant will need to be upgraded in the next five years to avoid overloading the system, according to a "confidential" report.
The report - commissioned by Kaipara District Council - said the worst case for the Mangawhai Community Wastewater Treatment Plant (MCWWTP) was that it would need an upgrade by 2026 with construction to begin in either 2023 or 2024.
The recommendation is based on a growth rate of 100 connections each year, according to the technical report - marked "confidential" - prepared by WSP for the council.
KDC works on the basis of between 70 and 100 connections a year in Mangawhai and there were 77 consented connections in 2019/20 financial year.
Mangawhai is the fastest growing area in Northland. Independent commissioners are currently considering a planning change to allow a Mangawhai Central Limited development that could see 1000 extra houses built.
If it gets the green light, the new development would put enormous pressure on the wastewater system.
The current controversial treatment plant came into operation in 2008 after a cost blowout, followed by years of legal battles between the ratepayers and council, and an investigation into the scheme by the Auditor-General.
In 2002, the scheme was tipped to cost $13.5 million, but the contractor went broke in 2004 and a $26m contract to service 2200 properties was signed in 2005.
The following year the project's scope was doubled to 4400 properties and the cost rose to $46 million.
Retired lawyer Clive Boonham, of Mangawhai, said KDC must disclose all information such as the capacity of the current wastewater system, what plans it had for the future, and to consult the community on such matters.
KDC general manager infrastructure services Jim Sephton said recommendations from the WSP report have been acted upon in terms of modelling, investigating disposal options and constructing the balancing tank.
He said KDC was using the report to develop its 30-year Infrastructure Strategy, Long Term Plan and Asset Management Plans.
WSP said if the number of wastewater connections was reduced to 70 every year, construction of the upgrades would need to start in 2027 with time allowed for early community engagement, planning, and consent application prior to that year.
The Mangawhai system currently discharges treated wastewater to a 110ha disposal farm about 12km away but due to the natural layout of the field, only 65.5ha of the land is available for effluent irrigation.
Based on the predicted growth rate of 70 connections per year, the disposal field will reach its capacity by 2032.
WSP has highlighted three options— discharge to either land via disposal field, to the estuary, or to sea via an ocean outfall.
"The upgrades will allow the accommodation of a larger number of connections, from the current 2000 to the available 7000 connections."
On housing development, submitters to the KDC's Mangawhai Spatial Plan favoured smaller lot sizes in future planning rather than higher densities such as townhouses and apartments.