However, the council responsible for protecting an old dump site next to the tranquil bay says the area in question is low risk, and it has no immediate plans to protect it.
The site is located behind an ever-crumbling coastal road, and residents claim it contains car bodies, batteries, plastics, oil, herbicides and paint.
Dolphin researcher Gemma McGrath, who has lived at Colac Bay/Ōraka since 2018, said the strip between the beach and wetland — which covered the old dumpsite — was getting narrower with every tide cycle.
"A worst case scenario would be nasty toxins like DDT and 245T leaching out into our marine environment, going through the food chain," McGrath said.
"Our ecosystem will be toxic, and our kai moana won't be safe."
Just what is contained within the ground is yet to be confirmed, but statements provided by 10 past and present residents paint a grim picture.
Some locals believe old herbicides are buried at the 50- to 60-year-old site.
In a written statement presented to the council, ex-resident Pete McKay pointed out agricultural chemicals in the 1980s were rife with ‘‘typically horrific chemicals like DDT, '2,4,5-T'", indicating some could be contained in the wetland.
She said Colac Bay/Ōraka was not just home to the eight or so Hector’s who frequented the area, but also orca, bottlenose and common dolphins.
"Colac was the first bay around mainland New Zealand to have a southern right whale give birth again, since the whaling days, when this species was targeted to near extinction," McGrath said at a Community Board meeting last week.
"Hector's dolphins have sharply declined over the past 70 years, due to accidental bycatch in fishing nets."
Recovery was a slow process because female Hector’s only had one calf every three to four years — and that was when conditions were good, McGrath said.
"They are true locals, having a small alongshore range of 30km-50km."
Because Hector’s along the southern coast depended on each other for breeding, if the situation turned south for the Colac/Ōraka regulars, it would also spell bad news for the Riverton/Aparima pod and those further down at Oreti.
"I’ve seen Hector's dolphins have their calves here. It's one of their core areas, right off the dump site.
"It’s right in the middle of their hood."
Someone who understands those concerns better than most is fellow Colac/Ōraka resident Carole Elder (Ngāi Tahu), who has made a living off the paua industry.
Elder described her back door as "a very spiritual bay" and a source of bountiful kai moana.
However, she feared what could happen if the old dumpsite was breached by the sea.
The land in question is owned by Ōraka Aparima Rūnaka, but the Southland District Council is responsible for the crumbling road which protects it from the ocean.
Elder has been tied to a group of locals battling the council for an outcome, but had distanced herself of late because she hoped for a peaceful solution.
Rūnaka kaihautū Riki Dallas said last week he wanted the council to provide evidence of what was in the site before jumping to conclusions about toxic waste.
But Elder said action needed to happen now.
"The people who signed their things [statements] aren’t lying, because what would be the point?
"They’ve seen it [toxic waste] go in there."
Elder said a worst-case scenario would be waste washing out to sea, poisoning kai moana and contaminating the popular local surf spot Trees.
Her message was simple: the rūnaka and council needed to sit down and come up with a solution that worked for all parties, beginning with putting protection in place where the site was most exposed.
The rock wall, which lines part of Colac Foreshore Rd, stops where it seems to matter most — right in front of the site.
"There’s whitebaiting, there’s floundering. We could live off our bay if we had to.
"You have to prevent it, not wait for it."
Southland District Council infrastructure and environmental services group manager Matt Russell said the council had commissioned three assessments of the Colac Bay site, which confirmed no intervention was required because the landfill was considered low risk.
The Foreshore Rd was first closed in July 2015 after being damaged by tides. It then reopened as a single-lane gravel road before closing again later that year.
The Hector’s dolphin is one of the world’s smallest, mostly found around the southern coast of the South Island.
Adult dolphins are about 1.5m in length.
Their conservation status is nationally vulnerable, according to the Department of Conservation.
- Matthew Rosenberg, local democracy reporter