Two families, who lived less than 100 metres from the Hume plant, say they repeatedly warned ACT environment authorities of their fears that Koppers was polluting the surrounding area.

The families told The Canberra Times that when it rained heavily, a grey or green foamy discharge would spew down the street, across their driveways, and into a nearby dam and a neighbouring horse agistment property.

The new claims about chemical run-offs from the site come after revelations that the groundwater beneath Koppers' former plant is contaminated with 2430 times the safe limit of the carcinogen hexavalent chromium.

Dave Holt, who lived in the Couranga homestead for 23 years from 1986, said the run-off flowed past his home and into a dam, where his children used to swim and catch yabbies. Mr Holt, who then worked as a land inspector, said he witnessed the run-off flow downhill into a nearby creek. ''It was just bloody ridiculous what they allowed them to do,'' Mr Holt said.

It is not clear what chemicals Koppers may have been discharging, and neither of the families claimed to have any knowledge of the concentrations or types of contaminants leaving the site.

In responding to the new claims, the ACT government said it tested soil and groundwater in the area surrounding the site three times, in 2003 at the Couranga sheep-dip site, 2008 at the Hume West Industrial Estate and 2011 at the ACT Horse Paddocks.

It said all of those tests found heavy metals in the soil to be within the acceptable criteria and consistent with naturally occurring levels in the capital territory.

Chromium, copper and arsenic levels found in the groundwater surrounding the Koppers site were also within the limits for stock and irrigation uses, the government said. It also said Koppers was permitted to conduct controlled discharges from the site at times, all of which it said was done in accordance with environmental safeguards.

But Mr Holt said he was never given any such assurances about the safety of the chemicals while he lived next to the site. He said he became so frustrated that he took his own samples of the discharge into the offices of environment authorities.

Mr Holt, who now lives on the Sunshine Coast, said he never got a response. ''Which I thought was pathetic and I just assumed it was still contaminated.''

A second resident, who lived by the Koppers site for 19 years, has also spoken of the frequent discharges from the site, which she said also ran across her driveway.

The woman, who did not want to be named, said the run-off was sometimes so strong that she couldn't get to the front gate of her home.

She said she saw ''deathly ill'' horses in the paddock directly opposite the Koppers site.

The long-time resident, who took Fairfax around the site of her old home on Wednesday, said she saw horses with discoloured tongues and clumps of hair falling off. There is no evidence linking those symptoms to the pollution.

The ACT government has assured the community that the groundwater contamination - a separate issue to the new run-off claims - is contained to the 20-hectare site, and that there is no risk to human health or the environment.

The ABC quoted two experts from the University of Canberra on Wednesday, who warned there was a risk the contamination would shift off-site, even if it was in an elevated pocket of groundwater, called a perched aquifer.

Authorities have not obtained any independent tests since 2007 to assure the public of their belief that the carcinogens would take hundreds of years to escape the site.

The government's belief is based on a comprehensive investigation of the site conducted by independent auditors in 2007 and further updates from Koppers and its consultants, despite the company shutting down and leaving in 2005.