Friends of Newcastle Ocean Baths, a group set up in recent years to stop the facility's pavilion being privatised, and regular baths users had expressed concerns about the rock bottom of the pool being concreted as part of its refurbishment, and how that would impact the level and spread of sand on top of the concrete base.

The existing pool floor is rock, and many believe it holds sand in place better than a concrete bottom will.

The sand floor allows for easier walking or wading in the pool. A concrete bottom, like the main pool at Merewether, is considered more difficult to walk across.

The group also questioned the "climate change" justification for raising the height of the pool and pool deck.

"We are concerned there has not been enough due diligence on the proposed concrete floor and that council are moving ahead with this proposal despite significant legitimate concerns," the group said in a statement.

"The climate change argument for raising the pool deck is also not convincing enough as far as we are concerned. We need climate change mitigation efforts to protect assets - but the right ones, informed by science."

A council spokesman said the pool floor was currently "undulating rock" and capping it would "allow for improved cleaning, water quality and safety for users".

"The sand bottom of the pool is formed by the occasional overtopping of the baths," he said. "This will continue even after the increase to the deck height, and sand will continue to accumulate in the pool."

The spokesman said the designs developed for the upgrade, due to commence later this year, were developed in consultation with a "community reference group, and expert advice on heritage, engineering, safety and coastal planning". The exact height increase is yet to be determined.

"It is essential the upgrade takes into account the 50-year design life and sea-level rise, ensuring the baths are resilient to increased inundation and high seas," he said.

"A range of planning advice is being taken into account when considering the height for the new pool deck, including the Newcastle Coastal Zone Hazards Study 2014, which shows likely coastal inundation hazards for the 2050 and 2100 planning horizons.

"The exact increase in deck height and depth of the pool floor is being determined in the ongoing detailed design process."

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