Friends of Newcastle Ocean Baths Incorporated spokesperson Tony Brown said the action was "a matter of last resort" after trying to speak with the council numerous times and councillor John Church's motion for an independent engineers report being voted down at the last Newcastle council meeting.

Council works to upgrade the century-old baths are due to start on March 14.

"We welcome the urgent, overdue works to make safe the baths and install a new pumping system to improve water quality, " Mr Brown said. "However, imposing a large sterile concrete slab over the existing rock shelf risks an immediate, irreversible and unacceptable harm to the unique heritage and indivisible interaction with the fragile natural environment of our much-loved baths.

"Numerous attempts by ourselves and some Newcastle councillors on our behalf to discover the reasons for controversial decisions and the evidence relied upon to support the same decisions have failed."

The baths user group is concerned the concrete floor could impact the ability of the pool to anchor and sustain its sandy bottom, which is a popular feature with waders.

However council insists the sandy floor will be retained once the concrete is installed, and will consider manually placing fresh sand in the pool after its annual clean.

The Hunter Branch of the National Trust (NSW) initially spoke in favour of the upgrade plans, but has now backed the heritage order application after hearing the baths user group's concrete concerns. The trust said the interim order sought to pause stage one works to allow Heritage NSW "to assess the potential state-significant heritage impacts arising from concreting the floor of the rock shelf".

Heritage NSW said the interim heritage application was being reviewed and the proposed works may not impact heritage significance, but this was yet to be determined.

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If an item becomes subject to an interim heritage order, all works will need to be approved by the Heritage Council while the level of heritage significance of the item is assessed.

Council said it will "fully comply" with any requests for information from Heritage NSW following the interim application but that there may be financial penalties for breach of contract with the construction firm.

"Given the late stage, this penalty is likely to be significant," a council spokesperson said.

"Any delay penalty cost will be borne by ratepayers and will impact City of Newcastle's ability to provide services and deliver other infrastructure works for the community."

But Mr Brown said council should "accept responsibility for the lack of effective genuine consultation and transparency in what appears as a rushed decision-making process with insufficient public and independent expert input."

Friends of Newcastle Ocean Baths and Newcastle council have both shown their support for the baths becoming state-heritage listed.

Newcastle council said it respected the "unique heritage value" of the ocean baths, which are listed as a Local Heritage Item within City of Newcastle Local Environment Plan.

A council spokesperson said they had engaged with the NSW Government to list the baths on the State Heritage Register, but had not yet received a response from the Heritage Minister or Heritage NSW.

Hunter Branch of the National Trust (NSW) chair Mark Metrikas said the trust commended council for its pursuit of a state heritage nomination, but had concerns the concrete floor could affect this.

"While we applaud council's stage one objective to improve water quality, user amenity and protect the heritage bleachers and boardwalk, concreting the pool floor is irreversible and has cultural heritage impacts that may endanger the prospects of attaining a state heritage listing," Mr Metrikas said.

The trust said heritage conservation required a cautious approach which respects existing heritage by "changing as much as necessary but as little as possible".

"Thus, the IHO also calls for an independent engineering review to evaluate the need to concrete the pool floor, and the impacts of raising the pool deck on the accumulation of a sandy floor," the trust said in a statement.

"Such a review should consider alternate options."

Mr Metrikas said a state heritage listing could guide council's stage two planning for the restoration of the baths pavilion and upper promenade, "to ensure the heritage of the baths is protected for future generations".

But council claimed the approved upgrade works were already consistent with the need to protect the baths' heritage.

"This included a Heritage Impact Assessment prepared by Barney Collins of EJE Architecture," the council spokesperson said.

"The stage one works have been supported by the National Trust as a part of the Community Reference Group through the development of the project, and were approved to commence in accordance with Part 5 of the Environmental Planning and Assessment Act."

The council spokesperson said engagement with the community, including representatives from Friends of Newcastle Ocean Baths, had been ongoing since 2019 regarding the scope of the upgrades.

"Substantial changes have been made to reflect the views of the community, including ensuring protection of heritage values," the spokesperson said.

However Friends of Newcastle Ocean Baths and the local National Trust claim the collaboration was mainly about amenities and aesthetics issues and did not include "substantive consultation concerning the concrete slab".

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