The NSW Department of Primary Industries Fisheries was alerted of the opening made on January 12 and Shoalhaven City Council rangers have been area carrying out investigations.

The opening of the lake was deemed illegal as it was not carried pursuant to the water trigger levels and council's Entrance Management Policy (EMP).

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The works created a 20-30-metre-wide channel, which has caused low water levels and poses a public safety risk for swimmers.

"This channel, between Swan Lake and the ocean, has resulted in the lake artificially opening at too low a water level and in unsuitable conditions," said Dr Michael Roberts, Shoalhaven City Council manager of environment services.

"This may lead to significant adverse environmental impacts on Swan Lake, such as low oxygen levels, associated fish kills as well as a significant swimming hazard."

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The channel, when opened by natural means or by council (under licensed conditions in accordance with the EMP), is located to the south adjoining the rock platform.

Swan Lake's sand bar is expected to re-establish in four to eight weeks depending on rainfall and tidal behaviour.

The entrance cannot be closed using any further artificial intervention, according to council.

In the meantime, signs have been posted by council to alert the public to the swimming hazard.

Dr Roberts reminded it is an offence to open coastal lakes, or lagoons, using machinery or by hand, without a licence and could result in penalties of up to $220,000 per offence.

Council operates an Entrance Management Policy for Swan Lake, and other coastal lakes in the Shoalhaven region, with licenses from various state government agencies.

This Policy allows Council to mechanically open Swan Lake to the ocean with an excavator to prevent flooding of low-lying properties surrounding the lake when the water level rises, typically following a large rainfall event, to a specific trigger level.

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