The government was not wrong to withhold information about potential toxic foam in the water supply from some of the households who could have been affected, the Ministry for the Environment's chief executive says.
Seven families living near air force bases whose water supplies have been contaminated with a toxic foam are still waiting to find out if they face health problems as a result.
The Defence Force previously admitted chemicals used in firefighting foam before 2003 had leached into nearby soil and water at the Ōhakea and Woodbourne bases.
The contamination was discovered in June 2015, but the first nearby households knew of it was December 2017 when the seven households found to have been contaminated beyond drinking water safety guidelines - set in April 2017 - were informed.
Studies have linked the toxic foam - a fire retardant used by the Fire Service, Defence Force, Marsden Point oil refinery and airports - to testicular and kidney cancer, tissue damage in the liver and a lowered immune system.
Free blood tests have been offered to five households near Ōhakea and two near Woodbourne found to have contamination above drinking water standards.
Ministry for the Environment chief executive Vicky Robertson spoke to Morning Report today about the contamination, the first time a government agency - including the Health and Environment Ministries, the Environmental Protection Agency and the Defence Force - has agreed to an interview since the contamination was revealed in early December.
She told Morning Report the reason people who might be affected by the foam had not been told in October last year - when, she said, the government first learned of the contamination from the Defence Force - was because the extent of the contamination was not yet known.
"In October we stood up a whole-of-government team. At that point in time we didn't know who was affected and what the impact of that was. So we didn't have any guidelines to say what is the impact of that here.
"We didn't know which houses might be above or below the drinking water standard."
The families whose tests had come back above drinking water standards had been told in December and offered an alternative water supply, Ms Robertson said.
"We talked to those families that were above the drinking water standard and we gave them alternative water supplies very early."
Communication could however have been handled better, she said.
"So what we would've done differently - and you're right ... is actually have a communication that is to the wider community so that everybody knows and they don't find out from their neighbour and wonder what's going on."
However, some level of foam chemical has been detected in drinking water at 41 properties - 19 in Ōhakea and 22 in Woodbourne.
One family of five who was living right next to Ōhakea Air Base said they had been told nothing of the toxic foam in the area and had found out through RNZ.
Ms Robertson denied the ministry's decision to withhold the information from some of those who could have been affected was wrong.
"What we're balancing here is going out to having a public statement about contamination, to actually going and finding out where the contamination is hitting and who is affected and giving people a bit more certainty about how it affects them," she said.
"We weren't wrong, we really wanted to focus on those families that we knew were above the drinking water standard.
"The one that you cited was slightly outside of that and they're okay, they have less traces of it.
"Now looking back on it, absolutely we could've done better about the communication."
The ministry wanted to do more testing this month, in April and in June, she said.
"We have talked to those communities about the test results."
Horizons Regional Council has called for the parameters for testing to be expanded to include more properties and waterways.
Ms Robertson said the Defence Force was involved in the all-of-government team - meaning the polluter that faces possible prosecution for the pollution is setting the parameters of testing for the pollution.
"We are looking at how far out this contamination has gone from the bases ... so the whole of government group is setting those parameters," Ms Robertson said.
She did not think it was a conflict of interest.
"We've tested the Rangitikei River, so that's another boundary. So I think, you know, at the end of the day, there is the professionals and the experts looking at where the water would go."
The results of the blood tests had been shared with the people affected but would not be made public yet because of privacy concerns, she said.
"Once we have some stronger testing we will be able to do that. The other this is we will be able to see where the contamination has moved to, so how far it has gone," she said.
There did not appear to be any effects for nearby dairy farms, she said.
"There is no general food safety issue here and that is the advice from MPI, they have tested milk and there is no traces of the contaminant in the milk."
"It's fine to eat leafy green vegetables, it's fine to eat eggs and all those things unless you are in this high-contaminated area."
The advice given to the seven households that were above the standard was more specific and tailored, she said.
"The advice to them will be different from people who are below the standard at the moment."
However, MPI said its testing found the chemical PFOS in three milk samples at concentrations below 100 parts per trillion. The chemical PFOA was not found.
"MPI toxicologists have calculated that an 82 kg adult male would have to drink 15 litres of milk containing 100 parts per trillion PFOS every day consistently, over a lifetime, to exceed the health based guidance value," MPI said in a statement.
What is the foam?
A fire retardant used by the Fire Service, Defence Force, Marsden Point oil refinery and airports. The foam contains the chemicals PFOS and PFOA - manmade chemical compounds which don't break down.
These are the most commonly used of the perfluoroalkyls chemical family, or PFAS, and diffuse extremely readily in air, dust, surface and groundwater, soil and sediment.
Human bodies get rid of PFOA and PFOS from their systems much more slowly than other animal species.
PFOA and PFOS are found in other things too, including some carpets, waterproof jackets, non-stick frying pans, and sneakers. Since 2010, some companies have been signing up to stop using these chemicals.
It has been banned in firefighting standards in New Zealand since 2006 and hasn't been used by the Defence Force since 2002.
However, Auckland Airport is still using a foam containing PFOA and the Environmental Protection Agency has announced an investigation.