Mangawhai Harbour's outgoing tide, a potential KDC wastewater discharge option. Photo Susan Botting
Kaipara District Council could potentially discharge treated wastewater into the ocean off the coast of Mangawhai at a cost of up to $37m.
The potential destination for the undisclosed amount of treated wastewater is part of a sewerage infrastructure expansion in Mangawhai - the country's fastest-growing coastal settlement.
The town's population of 4000 residents is forecast to grow beyond 15,000 people in 20 years' time but already the Kaipara District Council is grappling with demand.
Mangawhai's treated wastewater disposal is almost at capacity and major increased demand in sewerage infrastructure is expected over the coming decade.
KDC infrastructure planning manager Eros Foschieri said ocean outfall was one of three options on the council radar. The other two are discharging into Mangawhai Harbour or onto land.
Foschieri said they were yet to determine the amount of treated wastewater likely to be discharged into the ocean.
"Further detailed investigations and public consultation need to occur before we progress any further with investigating these options."
According to the council's website, the scheme's current disposal systems are at capacity, which limited any opportunity to take more wastewater into the treatment plant.
Already the number of Mangawhai wastewater scheme connections have gone up by more than a third between 1860 and 2018.
The scheme currently services 2500 connections, which are forecast to grow to between 5000 and 7000 connections within 10 years' time.
The situation has forced the council to look at the immediate option of discharging treated wastewater to the settlement's golf course. Wastewater would be pumped from the Mangawhai sewage treatment plant to a 500 cubic metre green steel holding tank in the golf club car park and spread at night.
Around 500 connections would be taken care of as a way to address immediate short-term needs.
The council also wants to boost its wastewater discharge capacity at Brown Rd farm, around 10km south of Mangawhai, where the material is spread across the land.
Kaipara Mayor Dr Jason Smith said an option could be converting the farm to a wetland. The council is looking at additional property suitable for the new discharge to land option.
Foschieri said a lot of work was needed to be done in order to decide which discharge option to go with, and each had its pros and cons.
"The best choice is what the community wants," he said.
The selected discharge option was important because it influenced how treatment plant expansion proceeded.
In the past, huge cost overruns for Mangawhai wastewater treatment had led to the Government putting commissioners into the Kaipara District Council.
Ratepayers were initially told when the scheme was first announced in 2003 that it would cost no more than $10.8m but had risen to $37m by 2009 and to more than $60m by 2013.
It was only last year when Kaipara District Council ratepayers finished paying off their allocation of the Mangawhai wastewater treatment plant debt blowout repayment.
Tangowahine-based councillor Victoria del la Varis-Woodcock warned that Kaipara ratepayers may now have to pay 50 per cent of interest costs on money borrowed for the future wastewater treatment expansion.
The KDC west coast/central ward councillor said the figure for the ocean outfall would undoubtedly increase from the potential $37m cost to as much as $70m.
She described the history of cost increases for Mangawhai's wastewater treatment plan as a 'debacle' and a 'woeful saga'.
Del la Varis-Woodcock also questioned whether "in the wastewater world" discharge to the ocean was "the right environmental choice" during a sometimes heated Mangawhai wastewater scheme presentation by Foschieri at the council's first meeting of the year.
She said the council needed to put the horse before the cart in the face of under-pressure Mangawhai infrastructure.
It needed to control the amount of development rather than spend big money on wastewater treatment expansion.
Del la Varis-Woodcock said Mangawhai developers' required KDC financial contributions paid towards the cost of infrastructure supporting their developments were 'woefully inadequate'.
She said the likely cost of Mangawhai golf course discharge had already gone from an original $1.5m to as much as $6.5m.
The cost of the Brown Rd land had initially been indicated at $361k but had increased to $14m.
Meanwhile, a new effluent storage or 'balance' tank currently being built has hiked in price from an initial budgeted $3.1m to $5.1m.
Next month the council plans to set up an informal community group that includes mana whenua representatives, technical specialists; local business and community group Mangawhai Matters, a councillor and KDC communications staff.