"We saw this in the March 1982 flood - the river near Canavans Knob flowed across the Lands and Survey farm and crossed into Docherty's Creek so if this goes ahead as planned you will lose the ability to create a safe and reliable connection between Docherty's Creek and the Fox Hills stretch of the highway. That area will become the new river bed."
Also at stake was a pristine wetland and kahikatea forest between the Waiho and Omoeroa Rivers, which would be inundated and lost, Milton said.
"These decision-makers in Wellington need to understand all the consequences of flooding the south bank. For example the proposed new road alignment they are talking about around the back of the dairy farm will be built mostly on the alpine fault which is very short-sighted."
Losing the airfield would be a big mistake as well, Milton said.
''In the December 2019 flood after the bridge and airfield were reinstated, there were hundreds of stranded tourists airlifted out from Franz Josef and if the Alpine Fault goes that airstrip will crucial for civil defence."
West Coast Regional Council chairman Allan Birchfield said he had received calls from landowners on the south bank of the Waiho, worried at the long-term proposal to let the river go.
"I don't agree with it. It's good land there, there's one dairy farm milking a thousand cows. It's very productive and it would be a loss to the country."
The landowners needed to be reassured that they would be fully consulted about any long-term plan to abandon their river protection, Birchfield said.
"If that was going to happen and I don't think it should, they would have to be bought out on a willing seller basis, it would be a huge amount."
In the meantime, the regional council would continue to maintain the south side stopbanks, with funding from the affected ratepayers, as it had in the past, Birchfield said.
"In fact we've got engineers down there today looking at it."
Westland Mayor Bruce Smith said if the south bank land were sacrificed, it would have to be on the basis of a fair market valuation and payout to the landowners.
"There are about 40 properties involved, there's the airfield, there's about 83 people. We've had a lot of meetings about this idea over the years; if you got rid of all the houses there would be a real housing shortage in the area - a lot of the staff in the tourist hotels lived there."
It was possible the farmland could be leased back on the basis that stock would move with the river as needed, Smith said.
"We would have to be very careful about removing any protection from the bridge to Canavans Knob - there's an old dump on there that's ten times bigger than the one that was washed out at Fox River."