The Department of Conservation is investigating reports that the nests and eggs of black-billed gulls are being destroyed by people driving onto a riverbed at Ashburton.
A colony of 3000 of the birds has been nesting there and Canterbury regional council says some nests have been destroyed despite signs that endangered birds are nesting nearby.
Senior biodiversity adviser Frances Schmechel says it is not the first time a motorist has driven on the river bed and destroyed nests and eggs of a vulnerable species.
River engineers are to block access to the site.
Disturbing protected birds and destroying nests can result in a fine of $100,000.
