Oil company Shell has contaminated groundwater in Taranaki using a firefighting foam banned more than a decade ago.

Foams containing perfluorooctane sulfonate (PFOS) were banned in New Zealand in 2006 but Shell was found to be using products containing the banned chemical when it became part of a nationwide investigation into contamination in March.

Testing found PFOS in foam at the company's Paritūtū and Omata tank farms near New Plymouth.

At a third site - the Māui Production Station near Opunake - the chemical was found in the groundwater but not in the foam, suggesting it was "likely historic, from previous foam that would have contained PFOS", Shell said in a statement.

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The groundwater at all three sites tested at above the health guideline for drinking water.

Shell Taranaki general manager Rob Jager said the company first became aware it "may have an issue with firefighting foams" in March.

SIMON O'CONNOR/STUFF Groundwater at Shell's Omata and Paritūtū (above) tank farms near New Plymouth has been contaminated.

There had been no further use of the foam and the company had worked with authorities to remove and replace the foams from the tank farm sites. That work was expected to be completed by mid-August, Jager said.

Two south Taranaki streams were also found to be polluted with the chemical and authorities were warning people not to eat the fish and eels.

The Taranaki Regional Council said the affected streams, the Oaonui and Ngapirau, were relatively inaccessible and iwi and local residents had been notified.

The council had referred its findings to the Ministry for Primary Industries for food safety advice.

TRC's director of environment quality, Gary Bedford, said the council decided to investigate after environmental PFOS contamination was found in other regions earlier this year.

"It's important to note that PFOS chemicals have been widely used in a range of consumer and industrial products," Bedford said.

"People are exposed to small amounts of some PFOS in everyday life, through food, dust, air, water and contact with products that contain these compounds."

The council was continuing to work with the community and companies involved in environmental investigations and keeping stakeholders informed.

Firefighting foam manufactured with PFOS and perfluorooctanoic acid (PFOA) were the standard in international aviation and the oil and gas industry from the 1970s until the early 2000s because they put out liquid fuel fires quickly, according to the Ministry for the Environment.

Since 2011, the import, manufacture and use of PFOS compounds has been banned in New Zealand, other than for specified, identified uses, such as laboratory analysis.

CATE GILLON/GETTY Oil company Shell has contaminated groundwater at three sites in Taranaki by using a banned firefighting foam.

In December last year, the Environmental Protection Authority announced it would investigate storage and use of PFOS and PFOA foams at airports and other locations.

EPA general manager of hazardous substances and new organisms, Fiona Thomson-Carter said last month the investigation had found PFOS-containing foams at various sites.

"The volumes and circumstances in which such foam has been found range from small amounts held in storage, to a few instances of larger volumes kept for emergency use. In these cases, safe disposal includes the need to decontaminate equipment and facilities, which can be an involved process," Thomson-Carter said.

"We are tailoring our response to the particular circumstances, and in some cases we are relying on undertakings that appropriate action will be taken by those responsible. We will subsequently seek verification that the agreed action has been taken."

As well as the eel result and elevated groundwater results at the three Shell sites, investigations in Taranaki found: