It presented a complex problem for aquatic scientists at Hydro Tasmania, who were eager to ensure the sustainability of the short-finned eel.

For the past two years they have used imaging sonar and acoustic tags to monitor the behaviour of the eels at the dam wall and power station intake to develop a solution.

Their answer: Australia's first eel bypass for downstream migration.

The $1 million bypass - inserted into a corner of the dam wall - was finished in June, ready for use during the migration season from December to April.

Hydro Tasmania aquatic scientist David Ikedife said the plan was to give eels an alternative to their current route, which involves either waiting for the dam to spill, or a treacherous journey through the turbines in the power station.

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He said their studies showed that eels would continually search near one corner of the dam wall for a way past, so it made sense to give them a bypass option at that location.

"It's been really carefully engineered looking at velocities and pressure changes, and the key biological criteria, so it's been designed to be really fish-friendly," Mr Ikedife said.

"The corner where the bypass is being located is chosen specifically based on the two-dimensional tracking with acoustic tags which gives these really cool tracks around the lake and we use that data to position our bypass."

After living their life in the South Esk, eels will attempt to travel thousands of kilometres to the Coral Sea near New Caledonia to spawn, before they die.

The juveniles then use the East Australian Current to get back to the Tamar, then the South Esk, where they use an elver ladder at Trevallyn Dam - a series of pipes added in the 1990s - to get upstream and into the dam, completing their journey.

Yet the lack of a downstream option had forced many to risk their lives at the power station turbines.

Mr Ikefide said the Australia-first bypass solution would be closely monitored during the upcoming migration season.

"It is a research and development project, so we won't build it and walk away," he said.

"We're going to use a piece of cutting edge technology called a sensor fish - that's like an electronic fish that measures pressure changes, rotational velocity and G-forces, so we'll put that through the bypass just to make sure that the as-built bypass meets all the biological criteria that we specified.

"In addition to those techniques, we'll be using traditional netting techniques at the bottom of the bypass to catch eels and check on their condition to make sure they're in tip top condition for their journey to the Coral Sea."

It also means more eels could start appearing at the First Basin during summer, but Mr Ikefide said this was nothing to worry about for swimmers.

"Hopefully the residents of Launceston might have some close and friendly encounters with some downstream migrating eels," he said.

Energy Minister Guy Barnett said it was great to see Hydro Tasmania using its aquatic expertise to improve sustainability outcomes for short-finned eels.

The eels are active at night, so passers-by will not be able to see them using the bypass during migration.