The 12,000 hectares of "lost land" in the Canterbury region is a complicated issue, Federated Farmers says.
The public land on the fringes of some rivers has gradually been lost to private development and absorbed into expanding farms from 1990 to 2012, according to an Environment Canterbury report.
Federated Farmers says farming beside a braided river is complicated due to boundaries and the river beds being poorly defined.
"The river can move but the boundary can not," the lobby group's water spokesman Chris Allen said. "It's a real concern... there's a lot of competing tensions from Department of Conservation, iwi, council."
Mr Allen says the report is likely based on satellite images, and he doubts anyone has actually physically checked the boundaries.
"Where's the title to the land? Have [the private farm owners] done anything wrong? That's the way we're looking at it. I doubt there's a huge amount of illegal activity going on."
The almost 12,000ha of lost public land defies belief, Forest & Bird said. The report, released last week, showed that while just over half of agricultural development had occurred on private land, a substantial proportion had been development of leasehold reserve land.
The encroachment was mainly within river protection reserves and endowment land along the margins of the lower Rakaia River, and in DOC reserves along the Rangitata and Waitaki rivers.
In social media posts Forest & Bird condemned the land grabs.
"It defies belief that around 12,000ha of land on Canterbury's river margins has been converted for intensive agriculture," it tweeted. "Our public land shouldn't be up for grabs and the agencies responsible for managing this land need to act when they become aware of the illegal development occurring."
Mr Allen disagrees, and says ECan needs to be clear about what the actual problem is.
"The report says there's an encroachment, but doesn't specify any illegal activity. There's a clear difference." - NZN