Government members voted in the Legislative Assembly to head off a performance audit sought by opposition environment spokeswoman Nicole Lawder, but a report will be prepared by the Environment Protection Authority by the first sitting week of next month.

Ms Lawder called for a report on contamination by the timber treatment plant, which resulted in a reading of 2430 times the safe limit of hexavalent chromium in groundwater beneath the site.

Despite the vote, Auditor-General Maxine Cooper might yet decide to investigate.

A spokesman for Dr Cooper said no decision had been made but consideration would be given to an investigation as part of the 2014-15 audit program.

Government ministers cannot direct the independent Auditor-General to conduct inquiries but they can request them individually or through an Assembly vote.

The spokesman said there were just seven performance audits each year and competing priorities from across the ACT.

As a result of amendments from Environment Minister Simon Corbell and the Greens' Shane Rattenbury, the EPA will report on its management of environmental authorisations for Koppers, its response to the 2007 audit that highlighted the high levels of contamination and poor compliance with testing, and its response to community concerns.

The US-based Koppers used a mixture of copper, chrome and arsenic to treat its logs at the plant until a decade ago. Public records show it failed to send frequent groundwater test results to government officials, a practice uncovered by EPA officers after Koppers stopped production.

Results from just two groundwater tests were received between mid-2002 and mid-2005, despite Koppers being legally required to send them quarterly.

Six lab reports were submitted between 1998 and 2002, despite the company receiving requests for results every four months. The EPA did not discipline Koppers nor seek to prosecute it for leaching the carcinogenic chemical into groundwater at levels well above the legal limits in its Crown lease and 2002 environmental authorisation.

Ms Lawder's motion required the government to provide public information on remediation of the groundwater within a month and for steps to be taken to ensure the situation was not repeated. Last week results from urgent testing at the site confirmed the chromium 6 had not spread from beneath the former Koppers plant. Officials said there was no risk to human health or the environment from the area.

Ms Lawder said more information was needed. ''This is about the failings of the EPA, the failings of the government and the potential for wider environmental harm into the future,'' she said.