Bobbing between tugboats and happy-go-lucky fishermen, the newest ship in Sea Shepherd's marine conservation fleet has moored in Darwin.

Dubbed the Ocean Warrior, the activist group's custom-built vessel has stopped for repairs and maintenance work after a campaign that uncovered thousands of illegally caught sharks in Timor-Leste.

Beneath the deck, the ship has undergone extensive modifications to better equip it for these kinds of campaigns, with massive fuel tanks filling the area usually reserved for crew quarters.

Captain Adam Meyerson put the Timor-Leste campaign's success down to the ship's custom design.

"The ship has engines out of the next-size-up patrol vessel, so it's the fastest one of these ever built. It also carries a lot more fuel," Mr Meyerson told ABC Radio Darwin's Lucy Periton.

At top speed, the vessel cuts through tides at speeds upwards of 55 kilometres per hour.

The $12-million Ocean Warrior was built with money from Dutch lotteries. ( ABC Radio Darwin: Jesse Thompson )

Watching and waiting

Speed is important and Sea Shepard said its raids all come down to timing.

Once a crew becomes aware of suspected illegal activity, they deploy a smaller, more agile vessel to investigate before first light breaks.

Mr Meyerson likens this to a stealth operation — waiting in the pre-dawn dark to scope out any sign of illegal activity on commercial vessels.

Beneath the ship's deck, seating has been swapped for massive fuel tanks. ( ABC Radio Darwin: Jesse Thompson )

If the crew arrive too late, they could be seen and any evidence of wrongdoing speedily disposed of.

But if the timing is right, Sea Shepherd can catch illegal fishers in the act and alert local authorities.

Sea Shepherd's tactics have been criticised as being too aggressive, particularly in their campaign to stop Japan's whaling in the Southern Ocean.

One tool in the Ocean Warrior's arsenal, nicknamed Fifi, is a high-pressure hose that pumps out water at a rate of 20,000 litres per minute.

'Fifi' the firefighting hose blasts sea water at high pressure. ( ABC Radio Darwin: Jesse Thompson )

"It's very convincing if someone's trying to run away or doesn't want to listen to you," Mr Myerson said.

"You point it in their direction and they start listening."

'Built for speed rather than comfort'

Day-to-day life on the ship is not such a high-stakes operation.

The Ocean Warrior's upgrades are "built for speed rather than comfort" and have come at the expense of a spacious crew quarters.

Isolated at sea for weeks at a time, Mr Meyerson and his crew come to know each other like family.

"It's a pretty unique experience in that you spend a lot of time with close quarters with other people, so you do have to get along," he said.

While moored in Darwin over the next month, the crew are keen to explore the Top End and surrounds — a vast area Mr Meyerson terms the "best last wilderness" in Australia.

"Certainly, Darwin has the advantage of having a very intact wilderness system," he said.

"It's probably one of the last in the world that's as expansive and untouched as it is in Darwin and the Gulf of Carpentaria."