Port Taranaki knew about groundwater contamination for more than a year before the public was told, documents show.
The same documents show that tests at the Maui petrochemical site near Opunake detected contamination up to 500 times safe drinking water guidelines.
Oil company Shell and the Taranaki Regional Council released a host of documents after RNZ inquiries into contamination from firefighting foam chemicals.
The contaminated water at test bores is not used for drinking.
The documents show the port learned in July 2017 about high levels of firefighting foam chemicals in one of several monitoring wells - 40 times drinking water guidelines - when it ran an environmental study before leasing the Omata Tank Farm.
This was six months before the public was first alerted to the contamination problem, when New Zealand Defence Force admitted at the end of last year it had contaminated water around several bases.
Port Taranaki did not tell the regional council about other findings until July 2018. The council told the public last month about contaminated sites.
Its chief executive Guy Roper said: "Port Taranaki has informed our regulators and will make no additional comment ... except to confirm that we will fully cooperate with any investigation."
The regional council referred RNZ's questions to the port company which it both owns and regulates.
The council's resource management director Fred McLay in a statement did not address the delay in public disclosure, instead pointed out the well with the high contamination was not on port land. It was on the boundary of land owned by Beach Energy near Centennial Drive.
Tests at wells on port land came up below guideline levels, he said.
"Therefore there was no need to report this," he said.
Beach Energy issued a statement but would not say when it was first told about the contamination.
"Ongoing annual groundwater and surface water sampling will continue," a spokesperson said.
Perfluorooctanesulfonic acid (PFOS) was banned completely in 2006, and the council said contamination was from back then.
Shell has admitted storing banned PFOS foam at both Omata and the nearby Paritūtū tank farm up until last month. But since 2006 it had only ever run one test at each tank farm using the banned foam. Even then it contained runoff, it said.
Test results from other sites show many are below the guidelines for drinking water and recreational water use.
These guidelines were adopted from Australia. Some US states have lower and others higher recommendations. A controversial official report in the US has pushed for much lower safety thresholds.
Some of the Taranaki test results are above the guidelines: The highest at 35 micrograms per litre is 500 times above the drinking water guideline of 0.7, detected at a bore in the northwest corner of Shell's Maui production station.
Eels in a nearby stream have high levels of PFOS. An advisory against eating them was issued last month, four months after Shell knew of contamination.
Other high results are from a bore at Paritūtū tank farm and from groundwater under the airport's now-defunct firefighting area.
Two local farmers told RNZ they were not concerned for their farms.
Regional councils in Auckland, Wellington, Bay of Plenty and Canterbury, said they had not asked companies to test potentially contaminated sites. The latter two councils had drawn up lists of potential problem sites.
* The documents released to RNZ are available by contacting the reporter.