Auckland’s water supplier has found low levels of toxic “forever chemicals” are still cropping up at a closed treatment plant.

The Onehunga Water Treatment Plant was shut down late last year after per- and poly-fluoroalkyl substances (PFAS) contaminants above drinking water threshold levels were found.

Watercare told RNZ at the time the likelihood of ongoing PFAS discharges was low.

However, four weeks of monitoring at the Onehunga aquifer in the past two months has found low levels of PFAS periodically in the raw water.

The agency had earlier suspended routine sampling at the plant. It has not said why.

International research on the health impacts of PFAS is somewhat inconclusive, with the latest from Brown University saying exposure in the womb is linked to a slightly higher body mass index and an increased risk of obesity in children.

The plant’s closure takes 18 million litres a day out of the city’s supply.

Early plans were being done to reopen the plant.

“We are keeping an eye on global trends and learnings around PFAS monitoring and testing,” a spokesperson said.

“We will be using the data we have gathered to inform the options for any future upgrades.”

Among the ways of getting rid of PFAS compounds - there are several thousand and they do not break down in nature or the body for centuries - the US Environmental Protection Agency recommends incineration, while activated carbon can reduce it leaching into water.

Another approach is mixing PFAS and additives with metal balls at high speeds.

-RNZ