What has fins, lives in the Top End and can fly?

Barramundi.

It may sound like the start of a bad joke (probably because it is), but it is a predicament Humpty Doo fish farmers have found themselves in as crocodile-infested floodwaters force the "evacuation" of around thirty tonnes of product.

With transport routes off the Arnhem Highway completely submerged, the barramundi must be airlifted to higher ground and on to market.

But 'Operation Fish Flight' is no easy feat.

Chilled barramundi are loaded onto crates before they are airlifted out of floodwaters. ( ABC News: Bridget Judd )

"We have to load the fish into nets [and] hook them onto the chopper," Humpty Doo barra Farm CEO Nick Preston said.

"Then we'll chopper the stock out for market consumption. It's much more cumbersome than what we normally do."

The operation is expected to continue into the week, as staff attempt to load and move around 40 helicopter trips' worth of freshly harvested fish.

From there, the stock will transported to the east coast in refrigerated trucks.

Staff watch on as a helicopter prepares to airlift around 30 tonnes of product to safety, ( ABC News: Bridget Judd )

It is a tricky operation, but one staff have endured before, after turbulent flooding cut transport routes in 2011.

"So we've got the benefit this time of experience," said farm manager Bob Richards.

"Even though there's a lot happening in a flood, we've got a really reliable group of people."

It is a welcome relief, with water levels not expected to drop until Monday.

A crate full of barramundi is flown out of floodwaters so that it can be sold at market. ( ABC News: Bridget Judd )

While the floodwaters are yet to exceed the heights of 2011, the operational challenges are not any easier.

Until the waters recede, staff must be ferried to and from work on a small tinny, navigating snakes, snags and everything inbetween.

"I haven't seen flooding like this before," said 'Nudge', an English native now working on the farm.

"We had a whole bunch of Death Adders swimming through the floodwaters this morning."

Thankfully, he quipped, there have been no crocodile sightings - yet.

Employee 'Nudge' has taken the flooding in his stride. ( ABC News: Bridget Judd )

As the first round of fish are flown out to market, around 400,000 fingerlings will be flown in from Melbourne to replace them.

"We'll be helicoptering those in too, because it'll put too much of a hole in our production in 18 months time if we don't," Mr Preston said.

"Similarly, if we can't get the market ready ones out to market, that puts a hole in the market in Brisbane, Melbourne and Sydney too."

After six to eight weeks, they will again be relocated to a feeding area - provided the flooding subsides.

Employees are being ferried to and from work on a tinny. ( ABC News: Bridget Judd )

It is these "feeding fish", or those over eight weeks old, that staff have particular concerns for.

As floodwaters rise, the oxygenated pumps sustaining them could go under, causing certain death.

In preparation for the worst, they are now being expelled through oxygenated tubes to a nearby pond.

"We're watching [floodwaters] at the moment and Marrakai is on the way down," Mr Preston said.

"We don't expect to see that water for three days, but hopefully after that we will be able to see some improvement."