A downpour of 75 millimetres in one hour on the evening of January 26 last year caused a newly-constructed containment dam at the Googong construction site to breach, with muddy water and silt flowing down the creek network into the Queanbeyan River.

Local Landcare volunteers and river-front residents at Wickerslack Lane were furious with the breach, and lodged a complaint with the Environmental Protection Agency (EPA) after water tests revealed a phosphorus spike up to seven times higher than the normal level.

Council, as the regulatory body, has been investigating the matter since, and general manager Gary Chapman told The Queanbeyan Age this week that Council had to "cover all its bases" in investigating the cause of the breach before considering prosecuting the developers of Googong.

"We acknowledge it's been 12 months (since the incident), but our solicitors and barrister have to be absolutely convinced that if we went to court, we would be successful.

"Otherwise not only would we be up for our own legal costs, but possibly those of the other side," Mr Chapman said.

"We have to be able to demonstrate negligence in a matter like this, and if it happened to be that the storm was so intense that it wasn't foreseeable, then negligence cannot be proved.

"And there's no sense in us prosecuting something that might only deliver up a very small fine and it costs us $250,000 to prosecute the matter," he said.

Meanwhile, Googong assistant project director Malcolm Leslie said the developers had increased water management precautions at the site over the last 12 months to prevent any further run-off.

"While the measures that were in place on Australia Day last year were designed and built in accordance with the relevant standards and approvals, the capacity of these was exceeded due to the severity of the storm," Mr Leslie said.

"Googong Township Pty Limited together with its consultants and Council has reviewed the practices on site at Googong and where feasible is endeavouring to implement temporary stormwater controls that exceed the standards so as to reduce the likelihood of their capacity being exceeded in the future."

Some of the measures include construction of major diversion drains to divert clean water from upstream catchments away from the construction zone and responsive management of the major pond in Beltana Park.

Mr Leslie said this would include sealing the low level outlet so that the pond can temporarily flood up to its 1:100 year design level and detain a greater volume of stormwater, as well as ensuring the pond level is kept as low as possible if storms are forecast and appointing a specific contractor to manage the pond levels.

And Council's general manager said the legal team would also have to consider whether the severity of the spill warranted prosecution.

"The Queanbeyan River suffered for some time after that, but whether in fact there was long term environmental damage is part of the consideration as well," Mr Chapman said.

"It was our understanding that after a month or two, the River was back to its normal condition."

The Queanbeyan Age also contacted Council's environment manager regarding any possible long-term effects of the spill on the Queanbeyan River, however no response was provided.