Palnackie residents are urging Scottish Water to take action to protect the village sewage works.
The utility company has deemed the £1.3m plant to be “at risk” after the adjacent retaining harbour wall collapsed.
It has been claimed that unregulated dredging and removal of protective silt undermined the stonework.
It is also now being alleged that a chain and padlock on a Scottish Water gate to its property has been forced.
Buittle Parish Community chairman Angus Campbell said: “Scottish Water padlocked their gate to stop access to their side of the harbour.
“But in the last few days the chain was cut and a new padlock was put on. I have emailed Scottish Water to make them aware of this.”
The latest development comes after the wall gave way and the banking between the works and water began sliding into the sea.
Locals blamed the dredging for undermining the plant, which was built in 2013 to replace ageing infrastructure.
Three giant chambers underneath process the waste water before it is discharged into the River Urr.
Mr Campbell said: “We are still struggling to have proper dialogue with Scottish Water.
“There’s no question they have to strengthen their sewerage system.
“No one else is able to do that work other than themselves – and they have to put it right. The harbour wall continues to be eroded as well.
“The whole harbour is a huge issue and people are not comfortable even going down there any more. We have a long haul ahead to get this sorted.”
Any dredging work requires consent from Marine Scotland.
The agency indicated it has no record of any application or licence granted in respect of Palnackie Harbour.
There was also no record of any communications regarding proposed dredging operations.
It is understood that Scottish Government officials are now investigating the matter.
Meanwhile, a Scottish Water has placed the matter in the hands of its legal team.
A spokesperson said: “Scottish Water was informed of an intention to carry out dredging in Palnackie Harbour by a third party. We responded to this highlighting a number of concerns given the proximity of our infrastructure.
“We need to maintain access to our assets at all times and any breach of site security and safety will be reviewed.”