Corrie Island - an internationally recognised nature reserve in Port Stephens - is under attack from dingoes that are gaining easy access due to sedimentation at the mouth of the Myall River.
The 164-hectare nature reserve is listed under the Ramsar Convention because it is an important breeding site for Little Terns and Pied Oystercatchers, both of which are endangered species.
Dingos have been regularly spotted on the island in recent months, triggering fears for the welfare of the island's inhabitants.
While they are naturally strong swimmers, the native dogs have been able to stroll across from Hawks Nest at low tide.
"We see them every week. The last one we saw had a couple of pups. There's usually another one that roams around by itself," Y-Knot Ferry Service operator Noel Gaunt said.
"They are certainly walking across. The river entrance was completely dry yesterday."
A NSW National Parks and Wildlife Service spokeswoman said the agency was actively monitoring wildlife on Corrie Island, and was aware that two dingoes were frequenting the Island.
"Dingoes are good swimmers and it's impractical to exclude dingoes from Corrie Island," she said.
"To this end, NPWS is trialling exclusion fences to protect nests from dingo and fox predation within Corrie Island Nature Reserve."
Nesting beaches in NSW may be marked by fences or signs, while parts of some beaches, including Corrie Island, will be entirely closed to visitors during the breeding months from August to March to allow these birds space and peace to raise their families.
"People are asked to keep away from nesting areas marked by signs or fences," the spokeswoman said.
"Domestic dogs are not permitted on Corrie Island Nature Reserve or National Parks at any time."
A series of drone photos published in the Newcastle Herald highlighted the estuary's dire state.
The natural river entrance is now almost completely blocked at low tide, which has resulted in one vessel having to be towed free and another three running around in the past week.
"We hit the bottom twice just yesterday," Mr Gaunt said.
Residents and business operators have warned the ongoing build-up of sand that now stretches several kilometres upstream will wreak havoc during the upcoming summer holiday season.
Of particular concern is the section of the river near the bush camp where hundreds of campers stay.
"People are already reporting they are hitting the sand up there," Myall River Action Group spokesman Gordon Grainger said.
"There will more than likely be a serious accident when you have people racing up and down the river who aren't familiar with the area."
Readings taken by the group at Tea Gardens show salinity levels of seven parts per thousand - healthy estuarine water is typically about 30 parts.