The Ashley-Rakahuri Rivercare Group called their actions on Saturday "glaringly predictable and disappointing".
Every year, 4WDs drove up and down the top half of the Ashley River as part of an unofficial drinking event known as Crate Day, the group said.
"Everybody's searched Facebook and we can't find any group. So when it's national Crate Day, we just go up there and wonder if it's going to happen," spokesperson Judith Hughey said.
The rivercare group put out a plea on its Facebook page for drivers to stay off the river, but the cars "unsurprisingly" turned up in the morning, she said.
The drivers ignored Department of Conservation (DOC) signs which informed them it was a restricted area during nesting season, Hughey said.
"It's just a classic case of a big group with big toys. They speed up and down the river, in and out of the water, as usual. Chicks wouldn't have time to get out of the way," she said.
The Ashley River was home to endangered bird species like the black-fronted terns, and Hughey said it was an important place for some birds which only nest in South Island braided rivers.
Another concern was the leaking of contaminants from cars, which were harmful to bugs and fish that the birds survived on, she said.
DOC biodiversity ranger Sandy Yong said enforcement was difficult in this situation.
"You almost need to have someone on site where a nest is, and demonstrate that a vehicle has gone over the nest, then be able to almost effectively stop the driver and get evidence directly," she said.
The department had increased signage at access points and nesting sites, and had tried to increase people's understanding about the value of these rivers.
DOC was going to investigate the damage to the nesting sites this week, she said.
"We are disappointed and we do have plans to review the impact this year, and are hoping to probably put in some additional measures."
