If the weir was to give way it could have a devastating effect on the local ecosystem.

The weir was built across Pourmalong Creek in the 1920s to supply the hospital with fresh water.

But the dam ceased to be used in the 1970s and the water it holds has since flourished into a diverse ecosystem.

As guardians of the land, Biraban LALC said they will do all they can to stop the weir from giving way.

The hole appeared only a few weeks ago and Biraban LALC chief executive officer Craig Foreshew said the group is now assessing the best plan of action.

The problem may be a very expensive one to solve.

Biraban LALC will consider all options, including asking the state government for assistance.

"The matter of the dam certainly requires a thoughtful strategic effort and collaboration with our NSW government in minimising any potential impacts to ecosystems and natural wildlife," Mr Foreshew said.

"We are presently negotiating solutions."

Biraban LALC deputy chairman Des Barton said if the weir was to give way it would impact on much of the native flora and fauna.

"We are seeking the support and backing of the NSW government to come up with a commonsense solution to the current problem in protecting the natural wildlife that has made [the dam] their home since the 1920s when the concrete weir was built," Mr Barton said.

Lake Macquarie City Council staff have inspected the site following a request from Biraban LALC.

The council will continue to provide environmental advice to resolve the issue.

"If a significant change in the water height within the dam/weir were to occur, it is likely that the vegetation communities upstream of the structure, which are accustomed to the current conditions, will be impacted," a council spokesperson said.

"Additionally, it is likely that the weir/dam provides habitat for numerous species ... that could also be impacted by a significant change in water level, should it occur."

 BIRABAN LALC said the damage to the weir is just one of many issues the group is tackling on the 750-hectare block of bushland the weir is located on.

They have previously dealt with illegal cow grazing and ghost tours on the property, along with people trespassing on motorbikes and in four-wheel drives.

Illegal dumping is a major problem, particularly on their Morisset land holdings.

Biraban LALC’s Craig Foreshew said the group was working to form healthy relationships with the community, local government and other relevant agencies to help stamp out dumping, along with other illegal activities.

“It should be all our responsibility to stamp out these actions that impact on all of us as Australians,’’ he said.

‘‘We have built up our monitoring of our surveillance systems, as this particular property is one of our largest.

‘‘It has been impacted on innocently with illegal activity that will be stopped.”