It 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.

The dam 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 ceremony at 5.30am and its completion marked on-site by local government representatives, other officials, designers and project workers, and the public at 9.30am 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.

The name of the dam will be announced at a dawn ceremony tomorrow to bless the completed project but Northland Regional Council is keeping that name under wraps until then. It was referred to as 'Kotuku' during its construction, after the Maunu street from which the site was accessed.