HIGH-ANTIMONY levels found in more than 300 Costerfield and Heathcote residents have been comprimised due to a suspected lab error.
Residents have concerns about the possible health impacts of the naturally-occurring metal in their system, which they believe is caused by the nearby Mandalay Resources mine.
A report commissioned by the Victorian government found elevated levels of antimony around Costerfield but determined that adverse health effects were unlikely.
However, the accuracy of high-levels of antimony found in residents has been thrown into question.
Department of Health senior medical advisor Danny Csutoros said questions were first asked when there was a sudden and dramatic fall in the number of tests above the reference range.
In the first few months of testing, 80 per cent of residents were above the reference range but this dropped to two per cent in late June and early July.
Mr Csutoros said brown-topped tubes - which were used for some of the tests - could have contributed to the elevated results.
"Antimony is used to make plastic and we think these tubes might have had antimony in them," he said.
"This could have contributed to the high levels but we are unsure of all the possible sources of false elevation and to what extent these factors have elevated the levels."
Health officials also haven't discounted the change of resident behaviour and dimished exposure as a possible cause in the drop.
"Residents have stopped drinking the tank water and have been asked to reduce their exposure to the dust in the area so this could have reduced their levels," he said.
"But I doubt it would have seen such a dramatic and immediate drop."
Mr Csutoros said the suspected lab error had provided comfort to some residents, but some maintained it was a conspiracy. "Ultimately we are getting to the bottom of it and we want people to be able to sleep at night."
He said people who had an elevated result were encouraged to get re-tested.