Firefighting foam contamination in Marlborough exceeds health guidelines for drinking water across an area of 200 hectares.
Contamination at lower levels is getting into the aquifer below Blenheim, a new Defence Force study concludes.
The contamination plume covers 800ha in all directions around the Woodbourne air base and extends up to 7km in ground and surface water east of the base.
The study forecast the plume would not spread very much as long as other things such as groundwater levels remained steady.
The study summed up two years of investigations as part of a nationwide inquiry into toxic and longlasting per- and poly-fluoroalkyl substances (PFAS) in water and soil.
One substance, PFOS - banned since 2006 in foam - and a less-studied PFHxS variant were the most prevalent chemicals. PFOA was also detected.
Spring-fed creeks at the headwaters of the Old Fairhall Creek and Doctors Creek, and water supply bores in the plume area, were worst hit.
"However, some contamination travels beyond the springs and into the confined aquifer beneat Blenheim, albeit with a reduced volume and at generally lower concentrations," the study by consultants Pattle Delamore Partners said.
Seven out of 203 private wells were exceeding the drinking water threshold. Households have an alternative drinking water supply.
Some fish in surrounding creeks had high levels of the chemicals, but the ecological risks have not been studied much and remained uncertain.
Most of the chemicals were in the soil, which was the source of the plume.
The stability of the plume depended on several factors such as "no significant change " in how much groundwater was pumped out.
"Health officials say there is no acute health risk, but a precautionary approach is being taken because the long-term effects are uncertain," the NZDF said in a statement.
"There is no consistent evidence environmental exposures to the low levels New Zealanders are generally exposed to will cause harmful health effects."
PFAS includes PFOA which was widely used in foam, and in Teflon and water repellents in the likes of carpet and raincoats, and which studies since the 1950s have linked to organ damage in animals - though these studies did not emerge until two decades ago, during US court cases.
However, research into human health effects has returned inconsistent and inconclusive results. The International Agency for Research on Cancer classifies PFOA as "possibly carcinogenic".