Traces of 1080 have been found in a Taranaki stream following drop of the pest control poison in Egmont National Park late last year.
The Taranaki Medical Officer of Health tested water samples taken from 17 sites. The testing was done within eight hours of the first aerial drop, then again four days later.
A single sample from Waiongana Stream measured 0.2 parts per billion, just above the limit of detection 0.1 parts per billion.
The Ministry of Health (MOH) guideline for safe drinking water is 2 parts per billion.
The Department of Conservation (DOC) said in a statement that when when the stream was tested a few days later no 1080 was detected.
Taranaki Medical Officer of Health Jonathan Jarman said he did not consider the results to be a public health issue.
Dr Jarman said all the tests required by the Taranaki District Health Board were clear of 1080. The positive result was found in a test requested by Taranaki iwi.
"We understand that very low levels of 1080 were detected in one of the rivers tested by DOC. This was not one of the tests we requested and has no impact on public health.
"1080 was not detected in any of the drinking water catchments and we do not believe that this is a public health issue."
In a statement DOC said the result posed no danger to human health.
"At the level of the MOH guideline (2ppb), a person must drink around 60,000 litres of water (more than a large milk tanker) in one day to have a 50 percent chance at a lethal dose.
"And based on research done by Māori researchers at Lincoln University, it appears that 1080 occurs naturally in puha at similar levels to sample from Waiongana stream."
As part of its Zero Invasive Predators (ZIP) programme DOC is undertaking a trial on Mt Taranaki to determine whether a modified 1080 regime can completely remove all rats and possums from a 1600 ha block.
It said if that was achieved, the new technique would reduce the need for repeated toxin use and provide a tool for progressing New Zealand towards its goal of being predator free by 2050.
Chairman of 100% Pure Taranaki, Chris Wilkes, said the poison's manufacturer warned it was toxic to aquatic life and finding it in the stream confirmed his worst fears.
"The fact that it's been present in the water means there's been a large number of pellets landing in that river at the top of the head waters.
"To find any kind of significant readings you actually need quite a few pellets to be in the water so it just shows me they were quite reckless with their application of the poison."
Mr Wilkes said 100% Pure Taranaki wanted people employed to trap pests.
Ngāti Awa ki Taranaki spokesman Robbie Taylor said it had placed a rāhui, or ban, on the gathering of seafood along the entire North Taranaki coast due to the 1080 drop.
Mr Taylor said he had noticed a lack of fish in the rivers and streams.
"If the Waiongana is infected then all those other streams and estuaries and creeks that run into the main rivers right round that mountain of the catchment, they'll be affected too."
Mr Taylor said the rāhu would stay in place indefinitely or until DOC stopped using 1080.
The Department of Conservation's next 1080 drop in Egmont National Park is planned for March.
The Waiongana stream flows through both the ZIP trial area and the wider DOC operational area.
DOC said water testing had been done after all three previous 1080 operations on the mountain (1993/94, 2002, 2010) and 1080 was not detected.