Te Rarawa chairman Haami Piripi said water from the reservoir would allow Te Rarawa to convert up to 400ha of dairy pasture into higher-yielding horticulture.
The iwi already owns the 212ha Bell's Produce market gardens, which, among other produce, grows mandarins and the country's first-of-the-season kumara.
Piripi said the area's first man-made lake would be ''good for everybody'' in the Far North, and was an example of what could be achieved when iwi, government and councils worked together.
Above all the reservoir would give certainty and allow the iwi to plan ahead, he said.
There was no point, for example, planting 100ha of kumara if the iwi couldn't be sure of having enough water keep the plants alive in summer.
Jones said the ''ingenious'' project would drive new types of land investment and new jobs, and commended the iwi for taking a risk when he was unable to secure the funding as a koha but had to make it a loan instead.
Jones also used the opportunity to announce PGF grants totalling $18.8 million for 11 Māori businesses from Northland to Stewart Island.
Iwi members taking part in Monday's ceremony included Lisa McNab, from Diggers' Valley, who said water storage was badly needed in an area that was increasingly afflicted by alternating drought and flood.
''This will support iwi development in so many ways,'' she said.
Tupehau is lined with just over 7.5ha of 1.5mm-thick welded high-density polyethylene sheets to keep the water in. It was built in a natural depression, with just one side needing to be raised.
The project would have been completed even sooner but for the difficulty of laying 10m-wide sheets of plastic in a wind that howls almost constantly off 90 Mile Beach.
