Heritage experts have decried as "madness" a state government push to retrench a team of stonemasons responsible for maintaining hundreds of iconic sandstone buildings across Sydney, including the Art Gallery of NSW, Government House and the State Library.

The government-wide push to outsource jobs, if applied to the stonemasons, will cost taxpayers more money, result in a loss of vital skills and prompt safety concerns if decaying buildings are not restored in a timely manner, say unions.

Unbeknown to most taxpayers, the NSW government employs a team of around 40 stonemasons, heritage roofers and scaffolders to restore, maintain and perform safety checks on about 650 state-owned schools, hospitals, courts, fire stations, railway stations, museums, parks and galleries in Sydney and regional NSW.

The team was established in the early 1990s in response to a market failure which saw a monopoly supplier charging inflated prices and amid uncertainty about the dwindling supply of precious "yellowblock" sandstone from which many of Sydney's most iconic buildings are hewn.

Known as "yellow gold", this type of sandstone is unique to the Sydney basin and was quarried for many years in Pyrmont, Darling Harbour and the eastern suburbs before urban development won out.

Oxidising to a deep honey colour, the stone was used extensively during the gold rush days to build many of Sydney's important public buildings.

The stonemasons now safeguard a stockpile of yellowblock on behalf of NSW taxpayers and chisel, saw and sculpt it out of a stoneyard in Alexandria. Stone from the stockpile was used in recent years to restore the Queen Victoria Building.

In June, the NSW minister for finance, Dominic Perrottet, announced a new push to outsource jobs from the Public Works department, of which the stonemasons are part.

"Some services currently provided by NSW Public Works that can be obtained from mature private sector markets will be transferred, sold or progressively discontinued," the statement said.

The future of the Alexandria stonemasons and their stockpile of precious yellowblock is now in doubt, with many fearing for their jobs.

According to their union representative at the CFMEU, Paul Connell, who worked at the stoneyard for 13 years, the loss of the small and highly skilled team would lead to higher prices for taxpayers, particularly if the stockpile of scarce yellowblock is sold.

"This stockpile is finite, and when Heritage Services and the stockpile are gone, so will the current healthy marketplace. We will return to having only one supplier of yellowblock stone in the market. The government will return to buying its raw material from a monopoly, and the single supplier of stone will have a competitive advantage in all subsequent processes.

"Having a large stock of heritage assets, the NSW government must maintain its presence in a small market. It needs to maintain a stockpile of stone until there is more than one supplier in the marketplace. It also needs to retain some capacity to actually process and execute work."

The director of advocacy at the National Trust, Graham Quint, warned any disbanding of the team of stonemasons would result in the loss of ancient skills.

"It's a disaster for heritage conservation because we would lose some of the best, award winning skills in Australia."

"You have only got to look at the number of awards they have won. These are the top people in the field. I don't think there's anybody in the private field that has the experience that they have. It's madness."

The conservation specialist and managing director of International Conservation Services, Julian Bickersteth, said the stonework program was put in place nearly three decades ago to protect Sydney's unique sandstone streetscapes.

"The government recognised that we had a unique component to our streetscapes of these great big buildings built of sandstone which weathered easily and required regular upkeep.

"The overview the program has brought and the way they protect Sydney's sandstone legacy has been very successful. Our building stock is in good condition as a result."

But that was no cause for complacency, Mr Bickersteth said: "This sandstone will continue to deteriorate. There's always going to be a need to look after Sydney's important public buildings and streetscapes."

Any job losses from the stonemasonry program would be "a real loss". "The great legacy of that has been a whole generation of stonemasons who have been trained specifically in the use of sandstone. That potentially is the biggest loss out of all this."

"There's nothing like it in the country."

Outsourcing the stonework program was unlikely to result in lower prices for taxpayers, Mr Bickersteth said. "I can't see the government is going to achieve major savings."

"I'd say the value is in the real estate. I don't think the program itself could be more economically delivered."

In response to questions from Fairfax Media, a spokeswoman from the NSW department of finance, services and innovation would not confirm the exact size of planned job losses, whether the Alexandria stoneyard will be sold or what provisions had been made to ensure lower prices for taxpayers and retention of important skills.

"NSW Public Works is repositioning to focus its expertise to the smart buyer/commissioning role and to transition out of services where the private sector provides a strong and contested capability," she said.

"The planning work for this transition has now commenced. The nature of issues raised in your questions will be considered and determined through the planning process."

"In the implementation phase, appropriate measures will be taken to support the government in obtaining value for money for the services it obtains from the private sector."

Mr Connell invited Mr Perrottet to the Alexandria stoneyard to see the craftsmen at work.

"Stonemasonry and heritage work are not well-remunerated or glamorous professions. Heritage Services and Heritage Design staff are generally paid less than many of their private sector peers. Staff remain because they take pride in executing work of the highest quality."

"To break up a specialist team that has decades of experience lacks any coherent vision of the future. Once it is dismantled and the corporate knowledge is lost it is not replaceable."