Because the Union Creek bridge is not wildlife-friendly, according to two of the area's activists.

Concerned Residents Group chair Debra Moore and Kiama Greens member Howard Jones have written to Kiama Council, NSW Roads and Maritime Services and Princes Highway upgrade contractors Fulton Hogan about a bridge widening at Union Creek.

Ms Moore and Mr Jones said Fulton Hogan would continue to use box culverts, or hollow, reinforced concrete boxes, to support the roadway.

"In terms of the ability for animals to access underneath, it's going to be worse because it's going to be longer and darker," Mr Jones said.

Ms Moore said ducks already avoided the culverts and used the road frequently during spring.

Despite the "ducks crossing" signage, not all were successful.

"A lot of water birds have been getting killed on the bridge for years, especially when you get a bumper season for ducklings," she said.

"We were here the other day and ducklings came out with the mother and we had to stop the traffic while they walked across the road."

Mr Jones and Ms Moore said they would prefer a more open design with a natural creek bed in place.

Mr Jones said Union Creek was the first natural feature to greet motorists driving off the Princes Highway.

"If it looks like a drain, it's going to get treated like a drain, if it looks like a creek, people will start thinking about it like that," he said.

The two were also concerned about barbed wire fences lining the cattle property boundaries along the upgrade's length, arguing animals could injure themselves on the wire.

"Ultimately the ideal solution would be the provision of proper exclusion fencing, which would prevent animals getting onto the road corridor in the first place," Mr Jones said.

He said the fencing could direct animals toward the highway underpasses for safe passage.

South Precinct chair Darrell Clingan said the group had not yet considered the matter, but he believed its members would support the action.

"It's all about improving thesafety of wildife in our community," he said.

Ms Moore and Mr Jones have requested a meeting with Kiama Council, RMS and Fulton Hogan.

Kiama Council was unable to respond before publication.