The popular holiday destination in Southland, home to about 60 permanent residents, is protected from the Southern Ocean by a sprawling rock wall.
But the sea is rising up to meet the town that faces Foveaux Strait.
Fearful an old dump could be exposed where the rocks give way to loose shingle, residents Alan and Deen McKay are leading the charge against a council they claim is disinterested.
“We want what the residents of Colac Bay deserve - that they can go to sleep at night and know the sea isn’t going to meet
The crumbling road is the only thing separating the ocean from the old dumpsite at the section of coastline where the rock wall peters out.
The McKays now fear if immediate action is not taken, the bank could be breached and waste washed out to sea with the next big storm.
Just what is contained within the land is unclear, but residents recently approached by the McKays to produce official statements have painted a grim picture.
About 10 past and present residents came forward with signed statements which have been presented to the Southland District Council and Environment Southland.
Resident Norman Cleaver sums up the claims: “Everything and anything was dumped here.”
Lynette Heath, who lived at Colac Bay between 1953 and 1970, says there were regular fires at the tip with “black billowing smoke”.
Her memory is backed up by another former resident, Benita Dudfield, in a letter previously sent to Environment Southland.
“For 33 years we ... endured the smell from fires lit at the site regularly by those dumping waste.”
“I can assure you the area is very toxic, and certainly the local people living there now have every reason to be concerned about the sea encroaching into that area,” Dudfield wrote.
In an email sent last week to key figures at Environment Southland, Southland District Council and Ōraka Aparima Rūnaka, Deen McKay said she had sought legal advice and stated the obligations of all three parties to ensure the containment is kept at bay.
The land in question belongs to the Ōraka Aparima Rūnaka.
Rūnaka kaihautū Riki Dallas said he wanted the council to provide evidence of what was in the site before jumping to conclusions about toxic waste.
“If it is [toxic], we’ll be looking at the council to come up with a solution to rectify any sort of environmental damage or harm that may be caused ... without pointing fingers, jumping up and down or accusing people.”
Last week, Southland District Council infrastructure and environmental services group manager Matt Russell sent an email to the McKays saying the landfill had previously been classified as "low risk".
He said it had already been acknowledged that, over the next three to five years, significant investment was required for maintenance of the wall.
He did not indicate if this included extending it to protect the dump site, and could not be reached for comment.
Mrs McKay will speak at an Ōraka Aparima Community Board meeting tonight about her concerns.