"People's biggest concern was that the plan suddenly appeared, so they're just hearing about it at the last minute. That's partly because it's been fast-tracked, but when there's a lack of information, people fill in the gaps," he said.
Other concerns included whether more horticulture would lead to greater use of sprays, and whether there was a risk of the dam failing and flooding the town.
"It's not that we don't want it, but everyone is saying, how about you ask us first?"
Resident Roy Shackleton said locals were alarmed that a large project was being rushed through without consultation.
"Everyone here's a bit shocked by the pace and scale of it. We just want to slow the process down so some thought and analysis can go into it," he said.
The proposal is to build Te Ruaotehauhau on what is now mostly pasture between SH1, Remuera Settlement Rd and Hariru Rd, north-west of Ohaeawai.
It would cover about 30ha, and hold 1.4 million cubic metres of water. Matawii, where work is due to start next month, will cover 18ha and store 750,000 cubic metres.
The trust's project manager, Andrew Carvell, said a resource consent application for Te Ruaotehauhau had been lodged with the Northland Regional Council, and all land owners who could be affected in the unlikely event of a dam burst had been written to. The trust had also applied for fast-tracked consent under Covid-19 recovery legislation, and was waiting to find out which process would apply.
Carvell said soil in the area was highly versatile, but its economic potential couldn't be tapped without a reliable water supply.
Possible uses included avocados, kiwifruit, market gardens and heirloom crops. There was some potato growing in the area already.
The dam, he added, would be built to the highest design standards. It would withstand an earthquake of magnitude 6.5, although Northland was not in a quake zone, and a downpour of 700mm in six hours. The record-breaking deluge that hit Kaikohe in July was 120mm in six hours.
Carvell said residents had raised concerns about land use changes.
"At the moment, it's rolling pasture with some small-scale horticulture, but if water was available, that would probably change, so some of the opposition is not to the infrastructure itself but to landscape changes," he said.
Another storage lake could be built just north of Te Ahu Ahu Rd, near Waimate North.