A stormwater retention dam in Maunu stopped around 300,000 cubic metres of water from flooding into central Whangārei during the weekend's storm.
Northland Regional Council (NRC) Environmental Services general manager Bruce Howse said the $11 million Hopua te Nihotetea, which opened in 2016, did its job in preventing the CBD from even more flooding from the storm that dumped more than 220mm of rain on the city in a few hours.
''We were very pleased with the performance of the dam and it performed as designed,'' Howse said.
''Without the dam in place we would have expected much more significant river flooding in the Whangārei CBD.''
He said the dam had about 400,000 cu m of water retained, which otherwise would have flowed into the CBD.
The dam has a capacity of 1,300,000 cu m of water up to the auxiliary spillways and 400,000 cu m is about 30 per cent of dam capacity by volume. The dam is designed to slowly release the water in the days after any storm.
Howse said calculations indicate that the dam reduced the flow in the Raumanga Stream from 114 cubic metres per second (cumecs) to 82 cumecs.
The river flow recorded in the Waiarohia Stream (at Lovers Lane) was the highest flow on record and peaked at 113 cumecs.
The river flow recorded in the Raumanga Stream (at Bernard St) was the third highest flow on record and peaked at 82 cumecs.
The 18m-high dam is the largest project of its kind undertaken by the NRC.
Officially called Hopua te Nihotetea, but known to many as the Kotuku Dam after the street it's accessed from, most of the time it will hold no water but is designed to come into its own in a large storm.
It works by slowly releasing trapped stormwater over a couple of days, reducing downstream flood levels by at least half a metre.
The dam will not completely stop floodwater as the Waiarohia Stream and tidal Hatea River also flow into the area, but it will slow the flow down the Raumanga Stream.