Transport officials planned to ask residents living near the Auckland Harbour Bridge if their properties could be tested for heavy metals, without telling them contamination had already been found nearby.
Documents show a letter was drafted and signed in 2012, inviting residents to be part of a highway study, but wasn't sent.
In the end, residents were never told about the contamination until this year when RNZ revealed it.
The New Zealand Transport Agency - Waka Kotahi - is now reviewing if bridge maintenance work could have caused health risks at their properties.
Auckland Council has admitted it should have told people about the discovery years ago, but said investigations showed there was no risk to human health.
The 2012 letter
The letter was on the verge of being sent to about 30 Stokes Point households, inviting them to have garden soil samples taken.
"We need your support and help," Waka Kotahi said in the letter.
It did not mention that the trigger for the testing was contamination found months earlier at a nearby reserve.
Waka Kotahi had discovered elevated levels of lead, zinc, other heavy metals, and a cancer-causing hydrocarbon at the north end of the bridge when it went to renew its maintenance consents in 2010.
The council investigated in 2011.
Some levels breached environmental thresholds, but not human health risk levels which are worked out differently.
But because of the discovery on reserve land, Waka Kotahi wanted to extend testing to include private properties. In the end, that never occurred.
To do the testing, Waka Kotahi had to ask residents.
The letter came to light in an OIA response.
It was dated 23 July, signed by a senior Auckland Waka Kotahi manager, and contained a consent form for locals to sign.
It told them the testing was part of a years-long highway impacts study, now expanded to include soil testing at several Auckland sites.
"The testing will not be noisy, and will be completed by our two testers," the letter said.
The samples from "homes like yours", would be analysed for contaminants left by tyres, brakes and exhaust.
This "will help the NZTA continue to improve our maintenance practices" and measure long-term impacts from work to ease traffic jams and pollution, it said.
Waka Kotahi's environmental manager at that time, Carl Reller, now retired, labelled the letter as "disingenuous and deceitful".
Documents showed Reller had concerns about health risks and wanted residents to be told and testing done so they could take action if need be - such as removing the top 100mm of topsoil.
Among the information the letter omitted was that bridge maintenance work was implicated in the contamination found at the reserve.
Up to 1.4 tonnes of zinc a year was released by paintstripping before 2013 when improvements began kicking in; a major project in 2008 released a lot of fine lead dust that contractors had trouble containing.
While soil samples had been taken at three other parts of Auckland, it was not from private gardens, and testing had only occurred for a 2012 study that came after the Stokes Pt discovery.
OIA documents show a scrap going on during 2011 and 2012 over whether the health risks justified alarming locals and the public by telling them.
Consultants in a 2011 study had reassured the Auckland Council there was no "unacceptable risk to human health".
Regional authorities advised not sending the letter
The Auckland Council and the local DHB's regional public health unit found out about Waka Kotahi's letter to residents after the date on the letterhead and were alarmed, documents showed.
When they learned the letter had not, in fact, gone out, the two bodies strongly advised the agency not to send it.
It was "laudable" to want to do the testing, but residents had to be given much more information first, they said at a 2012 meeting.
That would include telling people of possible "significant adverse impacts", such as that the tests might turn up pollutants above environmental thresholds, resulting in their properties being "tagged as contaminated".
This would stress residents, even if the pollutants found were just the normal consequence of living in a city like Auckland, the two bodies said.
Waka Kotahi undertook not to send the letter, but to revisit its management strategy.
What Waka Kotahi said
RNZ asked Waka Kotahi if the 2012 letter was misleading.
It replied: "The draft 2012 letter was not approved or endorsed for release by Waka Kotahi for a number of reasons, including that there was not sufficient evidence to confirm the need for soil testing of residential properties."
In 2013 Waka Kotahi ordered another health risk assessment.
This study concluded that health risks were probably low, but that "as there has been no actual investigation within the residential properties, there is significant high uncertainty with respect to the results of this assessment".
It recommended testing residents' gardens.
Nevertheless, Waka Kotahi told RNZ this was the study that led it to conclude "that residential properties adjacent to the Auckland Harbour Bridge would have acceptable contaminant concentrations, and no testing of residential properties was required".
Consequently, residents did not need to be asked about testing, and were not told about the contamination.
Some residents have told RNZ they are concerned, while others are not.
Waka Kotahi said its approach in 2012-13 took on board the council's advice to "be thorough in its assessments and planning to avoid unduly raising public health concerns".
Changes to bridge maintenance work since 2015 have radically cut how much dust and grit are released into the air, over land and the harbour.
Consents were being followed, Waka Kotahi said.
While it was confident about that, Waka Kotahi was "aware of residents' concerns relating to the potential contribution of historic bridge maintenance activities to soil contamination around Stokes Point Reserve" so was commissioning an external review due to report back in February.