Nelson city's climate champion Chris Cameron says planning for long-term sea-level rise of two metres might not be enough.
The council, which was among the country's first to declare a climate emergency, has released information that showed how coastal flooding could change under a warming climate, including a worst-case scenario of sea level rise up to two metres.
Cameron, who holds a PhD in Southern Hemisphere climate, said the science pointed to potentially greater change.
"Last time we had this concentration of greenhouse gases in the atmosphere, sea levels were at least five to 10 metres higher than they are now - some estimates have out them in the 20 to 25-metre range."
Nelson residents have been getting the detail on how their homes could be submerged by sea waters rising as much as two metres beyond the year 2150 if global temperatures kept rising.
Last week, 4500 property owners in Nelson City received letters telling them their homes and commercial buildings were at risk.
A public meeting late yesterday was a chance for residents of the coastal city to learn more from council staff and scientists.
In one room of Nelson's new Greenmeadows Centre, the Nelson Youth Choir practised a specially composed global anthem for 2020 - Paint Me a Rainbow Shining.
The music wafted next door where adults, a generation or three older, were hearing how their houses could one day sink beneath the sea.
Properties near the city's waterfront, within the central city and city fringes were now included in the the council's new hazard overlay zone. Long term it means they might face flooding. Short term it means the rising water risk will be included on Land Information Memorandums, or LIM reports.
Paul Bieleski lives a stone's throw from Tāhunanui Beach. He was not worried for himself, he said.
"Not really because... I'll be gone. I'm 86 and the statistics don't augur well for seeing it happen."
But he was worried about what it meant for the town's young people.
"When you look at the future it's desperate, and some of the maps show that - the future well ahead, but we don't seem to be able to look far enough ahead.
"Everything's so short-term."
Mayor Rachel Reese said sea level rise up to two metres was a worst-case scenario, if the climate continued to warm at its current pace.
"There are a range of scenarios that we can use and we decided to put all of them out. The "two-metre" scenario has been used by Tasman District Council and I think it's important that we align between the two councils.
"The boundary between us runs right up the middle of Tasman Bay, so I'd rather give all the scenarios than hold one back."
Suzi Bunting moved to Nelson from Wanaka six years ago.
She bought a property at Tāhunanui, and after doing her homework, was surprised to be among those to get a letter from the council.
"I'm in a row of six new houses that were built in 2014, and there were a lot of earthworks prior to the building to raise the properties, so they are far higher than our neighbours behind us."
Thames based coastal scientist Jim Dahm helped advise the council on where to draw the hazard lines.
He said coastal change was happening everywhere, but Nelson had one risk factor greater than others.
"Here you have very big tides, so you get a storm on top of that and you get very high sea levels. It's quite common here to get 3.8 to 4-metre tide ranges."
Luckily, the city had the 13km long, naturally formed Boulder Bank to protect it.
But Dahm said even that might not be enough in future.
"The Boulder Bank provides a lot of wave protection to Nelson Haven in the same way that Rabbit Island does for the Waimea Inlet.
"But none-the-less you get quite elevated sea levels within those bays from storm surge effect, which translate into the bay, so they're sheltered from wave effects but they still get quite elevated sea levels."
Reese said the uncertainty of climate change and sea level rise meant it was only possible to project what we know could happen, not what would happen.
The second public drop-in session runs tomorrow, 26 Thursday, November from 4pm to 6.30pm at the Trafalgar Park Pavilion.