The $8.6 million project repaired two sections of the wall that had threatened to topple into the sea because of saltwater penetration and erosion.

However, because of heritage restrictions, the government was not able to raise the wall's height, leaving it just a metre above high tide – undermining the government's message it will "remain secure for another 100 years".

A report by the Climate Council out on Wednesday says flooding in Sydney is already about three times more likely than a century ago, and will triple with every 10 centimetres of sea-level rise.

The survey on climate change and coastal flooding says expected sea-level rises of between 40 centimetres to 100 centimetres by the end of the century will globally make floods much more likely.

"Sydney is very vulnerable" to high tides and storm surges, said John Hunter, an oceanographer at the Antarctic climate and ecosystems co-operative research centre.

If carbon emissions that are heating up the planet remain on their current trajectory, places like Sydney and Hobart will be at risk of regular inundation as sea-levels rise. "One-in-a-hundred year [floods] by the end of the century will finish up being pretty much every day, with every high tide," Dr Hunter said.

"If we're very lucky, and we only end up with the low estimate for business-as-usual [emissions], then it will be every 10 or 12 days."

Sea levels have risen globally at a rate of about 3.2 millimetres a year over the past two decades. About 45 per cent of the increase has come from oceans expanding as they absorb more of the sun's heat trapped by greenhouse gases, with most of the rest coming from the melting of ice from glaciers and the polar ice caps.

According to the report, about 40 per cent of the state's 2109-kilometre coast is vulnerable to recession as rising seas – backed by storm surges – eat into sandy or muddy shores. Nationally, about 52 per cent of the coast is vulnerable, putting many fragile ecosystems at risk.

Almost 250,000 homes worth about $72 billion are vulnerable to a 1.1 metre rise in sea levels around Australia. NSW accounts for about a quarter of both tallies.

Road and rail, and commercial assets swell the national sum of at-risk infrastructure to about $226 billion – even before population growth and economic growth put more assets in harm's way, the report said.

Despite the risks, the NSW government under Premier Barry O'Farrell dropped its mandatory guidelines for local planning of coastal infrastructure. Developers had previously been required to take into account a sea-level rise of 40 centimetres by 2050 and 90 centimetres by 2100.

"You really need to be resilient because the further you get out into this century, the less certain you are about what it's going to be like," Dr Hunter said.