Down at the river mouth a 20-metre bank of sand stubbornly hems the river water in from the crashing waves beyond and a stiff sea breeze prompts a growing crowd on the cliffs above Moyjil to pull on their puffer jackets.

Every so often a wave will sweep up over the bank and into the estuary, and a sleek, arrow-shaped head will emerge from the brackish water.

The shallows are alive with dark, serpentine shapes, sliding over one another as they patrol the water's edge.

Occasionally one will seize the moment, wriggling out of the water to make a frantic dash across the sand, only to run out of puff as the water recedes again, forcing an arduous retreat.

This is what the crowd has come to see, the Hopkins River eel, waiting to make a break into the ocean and begin an epic, mysterious journey that scientific experts are still trying to get to the bottom of.

The southern short-finned eel (Anguilla Australis) is one of 19 species of freshwater eel across the globe. And like every other species of freshwater eel, it spends all but a few eventful months of its life in lakes and coastal rivers.

It has immense significance to many local Aboriginal groups, and has been a treasured food source for thousands, perhaps tens of thousands, of years.

Ancient eel traps at nearby Budj Bim have been carbon dated to 6600 years old in what many researchers believe is the earliest example of aquaculture.

Females are bigger than males, some growing beyond a metre and weighing more than three kilograms.

Their bodies are tubular and snakelike at the head before narrowing into a long slim fin towards the tail, which is what propels them through the water with a movement that is half slither, half wag.

While these strange creatures are well known to First Nations people, most other Australians have never heard of them.

Down at the Hopkins River mouth, many of the gathered crowd say they have lived their whole life in Warrnambool and this is their first glimpse of an eel.

"They are definitely an underappreciated fish," fish researcher Dr Wayne Koster says.

Since 2018, Dr Koster has been tracking the eels leaving the Hopkins River, helming a study for the Victorian government's Arthur Rylah Institute for Environmental Research.

The researchers knew the eels were breaking out of the estuary at a similar time each year, but they didn't know why or where they were going.

"What we did was collect eels that were around the estuary mouth, attached a satellite tag and released them into the ocean, tracking their journey to work out where they go and what happens along the way," Dr Koster says.

"We found out quite a lot of interesting stuff."

For one, the eels cover an enormous distance, swimming at least 2500km to the tropical waters of the Coral Sea.

"The tags we fitted them with were designed to pop off the eel at a pre-programmed time, which is between four and six months. So a couple of them popped off in an area not too far from New Caledonia," Dr Koster says.

He says it is entirely possible the eels still had further to travel.

"Of the ones we tracked to New Caledonia, one of them met its end inside the belly of a whale. We can work that out because the tags have a sensor that records the temperature around the eel. Suddenly we saw a spike up to around 37 degrees, which is indicative of a marine animal like a whale," Dr Koster says.

"The temperature reading stays like that for a couple of days and then goes back down as the tag makes its way back out of the whale's digestive system."

Dr Koster says many of the eels never make it to their destination.

"Quite a lot of them get eaten along the way by sharks and whales, so there's a really high level of predation."

Aside from temperature and location data, the trackers also send back the depth the eels are swimming at. It shows the eels move up and down in the water depending on the time of day.

"During the day they go down to about a kilometre below the surface where it's dark, which is probably an anti-predator tactic. Then at night they come closer to the surface, so they have this pattern where they're going up and down, up and down day-after-day and month-after-month," Dr Koster says.

And the point of this exhausting and often deadly journey?

Procreation, then death.

"It's a massive journey and it takes a huge toll on them. They die after they breed at the spawning grounds."

Scientists still know almost nothing about how the eels breed.

"About the only information we have about their breeding is handfuls of tiny larvae collected drifting hundreds of kilometres offshore," Dr Koster says.

Newly hatched, they are thin, transparent, leaf-like creatures.

They hitch a ride on the East Australian current, which crosses the Coral Sea and down the eastern seaboard, sweeping them thousands of kilometres back to Victoria.

The southward journey takes well over six months, by which time the eels have the strength to swim closer to the coast and into estuaries.

These "glass eels" are no longer leaf shaped, but are still transparent.

It is only once they start feeding in the rivers that they develop pigmentation and become known as "elvers".

They will spend the next several decades in freshwater, Dr Koster says.

"We've aged some using their otoliths, which is an ear bone that lays down annual bands like a tree with its rings, and some of the ones we've aged have been around 40 years old," he says.

Many venture hundreds of kilometres inland, growing fat on everything from fish, to yabbies and other small crustaceans.

Then something triggers a final metamorphosis into what's known as a "silver eel", the crucial preparation for their return voyage: the eyes enlarge to help them see in the ocean depths, while their snout becomes more streamlined and their skin becomes dark on top and their belly turns silver.

"That's potentially to avoid predators, which if they're looking down would see something dark and looking up would see something light," Dr Koster says.

Back at the Hopkins River mouth the river is still swollen, but none of the eels have managed to make it across the 20 metres of sand to freedom.

High tide has passed and so, it seems, has the eels' chance until tomorrow.

But as the crowd grows impatient an excavator appears, crawling down the dunes on the far side of the river.

Council has decided to open the estuary artificially.

Within 10 minutes it has carved a narrow channel and river water starts to flow gently through.

Eels loiter at the opening.

Two allow themselves to drift into the channel, but change their minds halfway down and wriggle back to their travelling companions.

A third is bolder, cruising all the way down the channel.

He - his size suggests it's a male - changes his mind at the last minute but it is too late and he disappears in the waves, becoming the first departer on this year's mysterious, one-way, tropical journey.