It has no name yet and is likely to be empty of water most of the time but the biggest dam construction undertaken by Northland Regional Council will be officially opened tomorrow.

The 18m high detention dam built in the valley between Maunu and Raumanga will hold up to 1.27 million cubic metres of floodwater during and after heavy rain.

Dubbed the Kotuku Dam, after the name of the Maunu street that gave access to the site, it will reduce flooding in Whangarei's central business district and the Raumanga Stream flood plain. Trapped stormwater will be released over a couple of days, reducing river swelling and downstream flooding.

Construction started in August 2014 on the $11 million dam which will be blessed at a dawn ceremony and its completion marked on-site by local government representatives, other officials, designers and project workers, and the public at 10.30am tomorrow.

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The dam is being paid for via the targeted Whangarei Urban Rivers Management Rate on more than 17,000 properties in the CBD and contributing catchment.

A name for the dam will be chosen after a consultation process.