Queanbeyan councillors have asked that the dam level be maintained at 80 to 85 per cent of capacity so that there is an ''air reserve'' to provide a flood mitigation buffer.

This has not happened and ACTEW Water says its hands are tied because ''Googong Dam is not designed or operated as a flood mitigation dam''.

Mayor Tim Overall said some residents ''do get nervous'' when the dam is at 100 per cent capacity and major wet weather is forecast.

This happened earlier this month when the Bureau of Meteorology forecast falls of up to 60mm in a 24-hour-period.

Falls of up to 15mm across the catchment are forecast for Friday. Googong has been hovering around the 100 per cent mark since the 2010 floods and most recently began overflowing across the top of the spillway on March 29.

About 3.275 gigalitres has overtopped the dam since then and flowed downstream to Queanbeyan and beyond.

This is just over 75 per cent of the four-gigalitre capacity of the original Cotter Dam.

ACTEW Water told Fairfax ''additional flows of 29 gigalitres from Googong Dam above licence release requirements'' had occurred since December 4, 2010.

This is just over 10 per cent of the ACT's total water storage capacity of 278 gigalitres and two-thirds of the 45.8 gigalitre reserve that has accumulated in the enlarged Cotter Dam since February 2013.

''It can be assumed the majority of this was due to overflows,'' an ACTEW Water spokeswoman said.

''[But] all flows are measured at Wickerslack gauging station which is four kilometres downstream [from the dam]. At times, especially during rain events, the flow at this gauge would be greater than the actual amount coming from the dam.''

There is no immediate cause for

concern downstream of the dam and even though Googong is full it retains limited flood attenuation capacity.

''There is some flood attenuation simply due to the existence of the dam, even when it is at full capacity level,'' Fairfax was told. ''The release capacity of the infrastructure is much less [by an order of magnitude] than what is naturally overflowing.''

ACTEW Water says there is no cause for alarm given the weather outlook and that, in any case, it liaises closely with the weather bureau. ''BOM have not issued a flood watch at this time. In the instance of a flood advisory being issued ACTEW Water provides both data and people to monitor the situation and inform the ACT and NSW Emergency Service Association.''

Cr Overall said he had led a public meeting to discuss ways to prevent future floods in Queanbeyan on December 23, 2010. That event had been considered a ''one in 20-year flood'' and caused an estimated $1.3 million damage, including washed-out roads, damage to parks and tonnes of water-borne debris that had to be removed.

''ACTEW Water undertook to look into the management of Googong in such a way that it would aid flood mitigation,'' he said. ''We suggested keeping Googong at around 80 to 85 per cent to provide an air buffer.''

Cr Overall said that while this might not stop a major flood it would buy time so that valuable equipment and assets could be moved to higher ground and locals would have time to prepare.