Former Bermagui resident of 40 years, Allan Broadhurst was a commercial fisherman.

He worked in water management for Eurobodalla and Bega Valley shire councils and spoke with many highly qualified government officials about the causeway over the years.

Mr Broadhurst was keenly interested in intermittent closing and opening lakes (ICOLs) like Wallaga Lake because "they are a vital link to everything".

"Most of the fish breed near Queensland but then they get swept down here into the ICOLs," Mr Broadhurst said.

"They all start in ICOLs so the water quality is vital to the fishing stock of the whole of NSW."

When the bridge was built in 1894 there was no money to build a raised bridge across the entire lake so they built a small bridge and a long causeway.

No one understood the impact it would have on the lake.

Mr Broadhurst said the causeway has blocked off the lake's main southern channel, Couria and Narira Creeks, to the ocean.

It has forced the water to divert north to squeeze under the bridge opening and then work its way back south to the ocean.

As a result "the lake is choking to death".

The lake has silted up considerably because it cannot flush properly between tides changing.

This was noted in the Wallaga Lake Estuary Management Plan of August 2000 where marine sedimentation rate was rated C or Fair, with a worsening trend, citing the causeway and more frequent artificial entrance openings.

It rated the volume of water leaving the estuary at ebb tide, as D or poor, again citing the causeway and artificial openings.

Mr Broadhurst said the causeway has impacted the lake's ability to regularly open and close naturally.

It now has to be manually opened by Bega Valley Shire Council (BVSC) and only stays open for weeks rather than months.

Again, the ocean's regular and proper flushing of the lake has been impacted.

The causeway periodically floods because the lake is not opening regularly.

Compounding the causeway's impact are agriculture and deforestation, leading to more erosion than would naturally occur.

It means that the contributory rivers and creeks bring soil, sand and sediment into the lake.

Mr Broadhurst said sometimes when heavy rain followed long drought, lots of vegetative material came down creeks into the lake, decomposed and took away the oxygen so that marine life unable to move to shallower water died.

When it last happened about eight years ago, it prompted the Wallaga Lake estuary management committee to raise the issue again and explore solutions like building big concrete channels but there was no funding available.

In 2012 the committee commissioned WBM Oceanics Australia to investigate in more detail the potential of a second opening because of the "widely held view" that the causeway was contributing to water quality problems in the lake.

It found that the removal of all or part of the causeway would not significantly improve flushing or water quality upstream of the bridge because the lake is mainly tidal.

Mr Broadhurst said theoretically it is tidal but due to the causeway tides no longer have much influence.

Only a fraction of the water that should be coming into the lake and going out into the ocean is actually moving.

The report said some parts of the lake would receive very little extra flushing if there was a second opening.

Mr Broadhurst said that showed poor understanding of the role of those back lakes which are naturally sheltered from the tides.

Those shallower, warmer waters support the juvenile fishes' growth and the lack of depth and structural coverage protect them from prey.

"Council used the lack of flushing of the back lakes as a valid reason not to go ahead with the plan [for a second opening] but the reality is that it is vitally important that those areas don't have extra flushing," Mr Broadhurst said.

"The lack of flushing in back lakes should not be an impediment for implementing the open causeway plan."