After almost 20 years of planning and a "gruelling" process to keep the project on track, the Waimea Community Dam, one of the Tasman District's largest-ever projects, is now under way.

Photo: RNZ / Tracy Neal

Supporters argued it was needed to soften the blow of increasing drought in the region, and to support a growing population.

But opponents remain worried about rates increases, and the debt burden for generations to come.

The concept of the dam in the Lee Valley has been almost 20 years in the making.

A company, Waimea Water Limited has been set up to take charge of construction, the cost of which about half was being covered by farmers and growers - the irrigators.

Waimea Water chief executive Mike Scott said it was the the role as much as the chance to return home that lured him to the region from an international career in the oil and gas industry.

And he could not be happier.

"I'm also driven by the fact I can be part of a legacy project for the region. I firmly believe we need this dam and it's going to be great for the prosperity of this region."

Not all shared that view. Opponents resented supporting a project that they felt would benefit a few, while supporters have long maintained the regional economy would be placed at great risk without a secure water supply.

Photo: RNZ / Tracy Neal

The project was almost stopped in its tracks late year when when it was announced the project costs had billowed, and councillors voted narrowly not to continue.

They reversed the decision when their concerns over who was to pay the increase were met.

Mr Scott said 80 percent of the workforce on the dam was local, but aspects of the construction required a level of expertise that was not readily on hand. He said experts from overseas and around the country had also been hired.

He said building the 53-metre-high dam was a complex engineering task, especially when taking into account the potential earthquake loads in the area.

"It's a great feeling to create jobs and support the local economy. The issue around the expertise we've brought in relates to specific, specialised dam engineering and construction."

Mr Scott said he was happy with progress so far, despite the late summer drought and fire hazard having restricted activity earlier in the year.

One of the project's most ardent supporters, long-time Nelson MP Nick Smith, said it was "tremendously exciting" to see the dam move into construction phase.

Dr Smith sought Parliamentary support last year to help remove another barrier to the project. A member's bill he introduced resolved the issue of access to Crown land for the reservoir, which received multi-party support.

Dr Smith said the dam's genesis was a severe drought in 2001.

Photo: RNZ / Tracy Neal

"It seems to have been an awfully long journey to have got here but there's no going back - construction has started and that's super exciting."

Dr Smith said the severe drought experienced by Nelson-Tasman this year had shifted the political mood to one of "just get on with it".

The mayor of Tasman District, Richard Kempthorne, said he could now leave the job, satisfied in the knowledge one of the region's largest-ever projects was now under way.

Mr Kempthorne said he was not so much elated as relieved.

"We've gone right through the 17-plus years of investigation ... a gruelling process to get to the decision to commit.

"It's just the knowledge this is such an important project for the region - and it's happening. It's so pleasing."

Mr Kempthorne was stepping down as mayor at this year's election, after 12 years.

The concrete-face rock fill dam will be about 53 metres high and 220 metres long. Construction is expected to be finished by October 2021. Once in place, the reservoir will fill up naturally over several months, with the final commissioning in February 2022, Waimea Water Ltd said.