An extra $40,000 is to be spent replacing 25 manholes at the failed Marsden City development south of Whangarei.

Contractors discovered more problems while repairing the subsiding systems - taking the total needed to fix the manholes to more than $1 million.

A report into the contract, presented to the Whangarei District Council (WDC) infrastructure committee on Thursday, said the contract value had already increased by $100,000 last August, but this had been "insufficient to deal with the extent of the issues which have developed as the contract has progressed".

The major issues contributing to the overspend included the installation of the PE manholes and damage discovered during manhole exposure.

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Other issues included the formation of concrete collars around all PE/concrete connections to ensure that the connection point was robust and sealed, which also required additional pumping of ground water.

Contractors also discovered holes which needed additional excavation, backfilling and sealing.

On top of that, stub flanges in the weirs of the PE manholes would be installed to allow the stormwater system to be emptied for future inspections and grates were needed under manhole lids for health and safety reasons.

The original contract sum of $960,706 had been increased by $41,342, bringing the forecasted total to $1,002,049, excluding GST. The funding was to be carried over from the Stormwater Projects line in the Long Term Plan for 2014/15.

The effects of acid sulphate soils were to blame for the subsidence. WDC waste and drainage manager Andrew Carvell confirmed that WDC has since started to include information about the soil type in all LIM reports.

Marsden City development was hyped to deliver 2200 homes with new businesses and industry. The project nosedived in April 2014 when it was revealed that sections of the waste and stormwater systems vested to the council had failed and would cost ratepayers millions to repair.

Court action against the firms responsible for designing and signing off the work, Cook Costello, HEB Smithbridge, Kennedy and Associates Ltd and Hynds Environmental, was ongoing.

It was estimated to cost $5.4 million to bring the at-risk waste and stormwater systems networks up to standard.

WDC had downgraded the value of the infrastructure to $2.5m, which had been gifted to WDC and valued at $11.2m.

The network was expected to last at least 80 years, but was now estimated in the region of 18 years.

Meanwhile, WDC was seeking to recoup about $8m from the companies it says were responsible for signing off the faulty infrastructure, including civil and structural engineers Cook Costello, which designed and oversaw construction of the development.

A defended High Court hearing between the WDC and Cook Costello regarding the failed development was set down for September 4.