Huge $25 million concrete wall designed to stop coastal erosion could wipe out beach completely

The beginnings of a huge $25 million concrete wall is billed as the solution to the devastating coastal erosion on Sydney's Northern Beaches.

Construction is well underway for the 1.3-kilometre seawall which stretches from Collaroy, to South Narrabeen.

Most of the multi-million-dollar bill is being bankrolled by around 50 homeowners while the remaining 20 per cent is split between Northern Beaches Council and the NSW Government.

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But the structure has divided locals, with many describing it as an eyesore, and there are concerns it may wipe out the beach entirely.

The wall is set to be seven metres tall, but three-quarters will then be covered by sand brought onto the beach.

"That sand, as soon as the waves increase, as soon as you get a storm, the sand will go," Professor Andy Short Geoscientist at the University of Sydney told 9News.

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"Eventually you'll end up with a seawall and no beach at all.

"What the council is doing is saying we prefer to protect a handful of beachfront properties rather than a very popular Sydney public beach."

But the Northern Beaches council says it's a necessary measure to halt and prevent further storm erosion.

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"It's just atrocious - and really disappointing," Brendan Donohoe from Surfrider Foundation said.

Some locals have described it as an "eyesore", but others said it was needed.

"I feel sorry for the people that live there - they need help and I think the seawall will work for them," one resident said.