Traces of a dangerous chemical used in plastic packaging, Bisphenol-A, or BPA, have been found in the Waiwhetu Aquifer in the Hutt Valley.
The aquifer supplies much of the water for the Wellington region, but that water is treated before being piped to homes.
A spokesperson for the Greater Wellington Regional Council said BPA got into water because it was a mobile compound.
It collected in water and percolated through soil into ground water, and was more commonly found in urban than rural areas.
Environmental toxicologist Louis Tremblay, from The Cawthron Institute, told Nine to Noon the chemical was ubiquitous, and very prevalent in modern homes.
It is a compound that is known to cause disruption in the endocrine system.
The endocrine system is made up of glands that produce and secrete hormones that regulate the activity of cells or organs. The hormones, which are also chemical substances, regulate the body's growth, metabolism and sexual development and function.
"So you don't want too many of those chemicals in your system, but then again it's not about raising a red flag, it's just a question of starting to think about the chemicals we're surrounded with, and the way we use them."
The amount found in the water was below European food safety guidelines, Mr Temblay said.
The chemical might not be a risk by itself, but there could be other chemicals in water.
"And when you start looking at those complex mixtures, I think at some point we need to at least have a level of caution."
Whether it was a concern for untreated drinking water from aquifers, would depend on whether the aquifer was in an urban area, or more rural.
Mr Temblay said BPA was found in a wide range of plastics.