Farmers and oyster growers who suffer damage from acidic water that has flowed from flooded swamps into the Manning River in northern NSW are unlikely to receive insurance payouts.

The sulphuric acid occurs naturally in swamp soils near Taree on the NSW mid-north coast. It is concentrated by land clearing.

“It’s a risk to farmers or a political risk that would go back to the local authorities around the clearing,” risk assessment expert Allan Manning, MD of LMI Group, told insuranceNEWS.com.au.

The swamps were cleared “between 50 and 70 years ago in the post-war years”, according to University of NSW Senior Research Fellow Will Glamore.

“If someone could be found liable, there might be a chance of some kind of recovery,” Professor Manning said. “But in this case it looks like it’s one of the risks of being a farmer.”

The acid pollution will take a month to clear, unless there is further significant rain, Dr Glamore told insuranceNEWS.com.au.

The phenomenon affects other rivers in the region, including the Macleay, Richmond and Clarence.

Acidification has caused oyster farming to plummet in the Manning River in the past 60 years, Dr Glamore says. It can also affect milk production.

Remediation works are being installed in some areas.