THE Pacific Hwy upgrade between Grafton and Iluka will only be built to the one-in-20-year flood mark.
A spokeswoman from the Department of Roads and Maritime services confirmed the plan on Wednesday.
She acknowledged the highway would be built on a flood plain.
"The Pacific Hwy upgrade between Grafton to Iluka extends to part of the Clarence River floodplain, including the upper areas of the Coldstream River and Shark Creek basin," the spokeswoman said.
"The upgrade will include building bridges and drains to allow water to safely flow under the road while minimising flooding impacts.
"The bridges and drains have been designed to ensure minimal impacts occur on both upstream and downstream properties.
"The highway has been designed to allow two lanes to remain open to traffic even in a one-in-20-year flood event.
"The road will be built to a standard specification and will be treated to avoid erosion which will improve flood immunity to the Pacific Hwy."
The Clarence Valley has experienced three floods in the past five years that exceeded the one in 20-year mark.