Muddy river could be 'crystal clear' lake

Tasmania's silted Tamar River could be turned into a clear freshwater lake four-fifths the size of Sydney Harbour, a lobby group says.

The murky Tamar is as much a part of Launceston as its favourite son, Ricky Ponting, but the group says a $320 million barrage near its mouth would produce a water-lover's playground.

"It will be virtually crystal clear," Tamar Lake Inc spokesman Robin Frith told AAP.

"Instead of having a visual depth of about six inches ... you'll be able to go down two or three metres."

The group this week released a KPMG economic impact report that says the project would create 850 jobs during its three-year construction period and the same number in agriculture and tourism.

It would generate $315 million for the state's economy in its first three years of operation, the report says.

Other benefits would include stopping the effects of predicted sea-level rise of 0.8 metres lapping the streets of Launceston, Mr Frith said.

The retired engineer is calling for a full feasibility study to be completed by government, which would include a submission to Infrastructure Australia.

He says Singapore solved an identical problem with a similar solution.

"(Singapore River) used to be just a muddy bank with old junks parked on the side," he said.

"The lake now supplies fresh water to the city of Singapore, whereas before they had to import fresh water.

"Now it's an aquatic paradise."

Governments have promised $125,000 for 3D modelling of the proposal to be done.

The lake would be 60km long and contain 405 gigalitres of water, 80 per cent of Sydney Harbour's 500 gigalitres.

It would be seven times the size of Canberra's Lake Burley Griffin.

Boats would come and go from the river through two locks.

The barrage would stop the incoming tide causing the silt choking the waterway, Mr Frith said, but the project would also rely on water authorities improving sewage disposal in the river.

Tamar Lake Inc has raised $420,000 and has completed studies on the proposal's technical and environmental viability.