Two key parts of a multimillion-dollar upgrade of the Awanui flood scheme have been completed.

The Northland Regional Council is undertaking a staged $15-million upgrade of the decades-old scheme, with work expected to run over several construction seasons until 2027.

Designed to protect urban Kaitaia in a "once in a century" flood and a one-in-20-year event in surrounding rural areas, the work is one of several key projects at the heart of the council's Long Term Plan 2018-2028.

About 40 councillors and members of the Awanui River Flood Management Working Group, landowners, contractors, tangata whenua, community members and others gathered at Te Ahu in Kaitaia recently to officially mark the successful completion of two key pieces of scheme work.

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Carried out by local firm Kaitaia Contractors, the first project involved $200,000-plus of repairs to a roughly 500-metre stretch of undermined stopbank behind Te Ahu.

The second almost $300,000 project saw construction of a new emergency spillway opposite Kaitaia's slow-moving Bell's Hill slip site.

Those at the event heard the council had over the past few years been working its way through the most urgently needed scheme repairs, with the new works delivering much-needed improvements, as well as effectively future-proofing it.

The council has been monitoring and managing the Bell's Hill slip site for many years, worried it could potentially slip into – and block – the nearby Awanui River.

Several speakers described their relief the new spillway was now in place, also expressing their gratitude to local landowners and the wider community for the backing the scheme upgrade generally had enjoyed to date.

Built on the 14,600sq m former Firth concrete plant site, the spillway will probably carry floodwaters only once or twice a year, but crucially, should the Bell's Hill slip ever collapse unexpectedly and block the Awanui River's existing flow path, it should be big enough to carry the river's entire flow.

Future flood risks in the scheme area will be mitigated largely through planned extensive modifications and improvements to stabilise existing stopbanks.

Once completed over the next few years, these will allow the river to carry up to 15 per cent more floodwaters.

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Without the added protection of the upgrade, a 1:100-year flood event in urban Kaitaia could cause tens of millions in damage and put lives at risk.

Seventy per cent of the upgrade will be funded by ratepayers Northland-wide via a regional flood infrastructure rate the regional council introduced last year, with the balance funded locally through the targeted Awanui River Management Rate.