Work should be completed today on shoring up a slip near the inlet channel of the Maunu stormwater detention dam.

About 450sq m of rain soaked loose ground and trees 50 metres upstream slipped in May.

While it did not affect the dam's structure, engineers recommended the repair to stop material washing into the inlet channel and impeding the release of water, Northland Regional Council (NRC) Environmental Services general manager Bruce Howse said.

The repair bill has added $80,000 to the $11 million cost of the dam, being footed by 17,000 Northland Regional ratepayers with properties in the downstream area that now benefits from flood protection.

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The repair bill will extend the 20-year term repayment by three months, based on no variations to future costs and revenue estimates.

The 18m-high dam, capable of holding up to 1.27 million cubic metres of floodwater, was opened a year ago and is the largest project of its kind undertaken by the NRC.

Officially called Hopua te Nihotetea, most of the time it will hold no water but will come into its own in a large storm. It works by slowly releasing trapped stormwater over a couple of days, reducing downstream flood swelling by at least half a metre.

The dam will not completely stop floodwater from affecting the CBD as the Waiarohia Stream and tidal Hatea River also flow into the area, but it will slow the flow down the Raumanga Stream and other tributaries.