An unknown quantity of sewage-contaminated fill has been dumped by contractors without permission, just metres from the Ohau River.
A member of the public said he saw a large amount of fill that had been recently dumped upstream of the town's water treatment plant, which had old sewage pipes sticking out of it, and was accompanied by an overwhelming smell.
The man said he had previously seen a large contracting firm taking a truck up to the site.
"[They] have been carting it in there," he said.
"It's urban waste, a bluey colour."
The man said he had worked in the building and construction industries and was familiar with the appearance of the fill and pipes and the smell.
He had contacted Horowhenua District Council last Friday, and the Ministry of Health's public health protection unit on Monday, which had at that time not yet heard of the issue.
They confirmed they had instructed HDC to carry out an investigation.
Horowhenua District Council Water and Waste Services manager Paul Gaydon confirmed two truckloads of sewage-contaminated fill from Bartholomew Road in Levin had been dumped at the site by the contractor, along with some road work waste from Fairfield Road.
He said they were never given permission to dump the material there.
"It was a mistake," he said.
Gaydon said the contractor had been asked to remove the material on Friday, however they removed some, but covered other material over.
HDC Group Manager Infrastructure Services Gallo Saidy also said the contractor did not have permission to dump the material there, but thought it was an error on the part of the truck driver.
The site was an area where the dumping of other clean fill had been permitted before.
He said on Tuesday that most of the material had now been removed, although the witness said it hadn't and the only tyre marks visible at the site could not have belonged to the type of vehicle needed to carry that out. It had been covered up, the person said.
The Horowhenua Chronicle visited the site on Monday evening, and much of the material still seemed to be present but covered over.
Gaydon confirmed on Tuesday morning that some of the fill had been removed by the contracting firm, but that some remained and had been topped with clay.
He said he didn't know if or when HDC had first been in communication with the Ministry of Health over the issue, but the authorities had been in email correspondence.
For the material to be contaminating the river, it would have to be flowing through the ground, and the council hadn't seen evidence of that.
Saidy said HDC had investigated the situation, and said the town's water supply was not in danger.
"In terms of contamination of the water supply, it has no direct link at all," he said. He said it was important not to alarm the public unnecessarily. Because the water take was about five metres below the river gravels, it wouldn't be affected.
The contracting firm was unavailable at time of print.