The decision by the council has been welcomed by the council, while Friends of Newcastle Ocean Baths spokesperson Tony Brown said the group, which applied for an interim heritage order, was "bewildered and surprised by the decision".

The application was the latest effort by the group to stop the council pouring concrete onto the existing rock base, saying it's better than any artificial addition.

The City of Newcastle said the heritage council decision meant the baths restoration could proceed without delay.

"Heritage NSW, in making its determination, noted Newcastle Ocean Baths construction is similar to that of all State heritage listed baths within "NSW," a council spokesperson said.

"The determination noted that the construction method of pool floors for these types of ocean baths, such as sand, concrete or rock, is not integral to the heritage values for ocean baths in New South Wales."

The council quoted the Heritage NSW letter saying there was no threat to heritage values from the upgrade.

"The City hopes Heritage NSW's decision to refuse the Interim Heritage Order brings an end to the continual stunts and misinformation by the group Friends of Newcastle Ocean Baths who have for more than two years attempted to prevent this much needed upgrade from occurring."

More on this issue: Council confident ocean baths changes will be no problem

Mr Brown disputed the accusation of "stunts and misinformation", saying the group supported the upgrade, but not the replacement of the existing "sandy bottom" and natural rock shelf.

He said the group worried the council's "demeaning and inaccurate" language could influence "the independent government bodies we have sought assistance from as a last resort".

He said waders preferring Newcastle baths to the "slimy" feel of the bottom of the concrete-bottomed Merewether baths showed the merit of their argument. He pointed also to the letter lord mayor Nuatali Nelmes had released before December's council elections asking the state to upgrade the baths from local to state heritage significance.

The council said the pool edge would be raised to cope with any rising sea levels, saying the baths would not "survive another 100 years" without the extra height.

"Without concreting the rock floor that currently exists at the baths, the increase in the height of the pool edge would make it too deep for most people to be able to walk in," the council said.

"By placing a relatively thin concrete base over the rock floor, the existing depth of the pool can be retained."

Letters: Ocean Baths group fires back at Paul Scott over 'branding critics whingers'

The council said a "15cm increase in the edge of the pool" would not stop sand flowing in from the ocean, but if it did, the council would add sand.

Mr Brown said the fact the council was now saying it would add sand to the new bottom of the pool confirmed the group was correct.

The heritage order rejection was signed by the executive director of Heritage NSW, Sam Kidman.

The baths have been fenced off for work to start, with the stage-one works scheduled to finish next year.

The council says the $14.5m first phase focuses on the pools, lower promenade and pumping system, addressing issues through community engagement and engineering advice.

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