Anchored in calm blue waters off Wedge Island Nature Reserve in the Great Australian Bight, a ship is approached by a pod of eight curious dolphins before they swim away.

One hundred metres below us, a kelp garden teems with colourful life.

Hundreds of timid blue devils with electric blue spots duck for cover in rocky crevices.

Bright red harlequin fish, known as "Chinese lanterns" because of their distinctive colour and shape, amble across the sea floor.

A harlequin fish (othos dentex) at Seven Mile Reef, Kangaroo Island, in the Great Australian Bight. ( Supplied: Richard Robinson/Greenpeace )

Although we're thousands of kilometres from tropical waters, the colourful fish are swimming among a rocky reef adorned with fluorescent corals, stretching in complex tangled geometries toward the surface.

"It's a beautiful mish mash of these species from the east and west," marine biologist Georgina Wood says.

She is currently completing her PhD in marine biology at the University of New South Wales.

The reef below us stretches from this spot in the Great Australian Bight all the way to the northern edge of New South Wales in the east, and to about a third of the way up Australia's west coast.

Despite its relative obscurity, it is estimated to generate $10 billion each year for the Australian economy through fisheries and tourism. That's about 50 per cent more than its more famous cousin, the Great Barrier Reef.

It is becoming known as the Great Southern Reef.

A leafy sea dragon (phycodurus eques) is photographed on the Greenpeace, Making Oil History Rainbow Warrior tour. ( Supplied: Richard Robinson/Greenpeace )

Now, part of this unique environment could be developed to become one of Earth's biggest offshore oil fields, with a string of about six Australian and international oil companies lined up to drop exploratory drills into its sea floor in search of fossil fuels.

A southern blue devil (paraplesiops meleagris) swims at Kangaroo Island in the Great Australian Bight. ( Supplied: Richard Robinson/Greenpeace )

"We believe there's a very significant resource of oil and potentially gas in the Great Australian Bight," director of external affairs at the Australian Petroleum Production and Exploration Association (APPEA) Matthew Doman says.

The Great Australian Bight is a large area off the south coast of Australia, and drilling there is unlikely to occur directly on the reef.

BP, the oil giant that initially led the charge to drill in the Great Australian Bight, said in 2013 it could one day be as big an oil region as the US Mississippi delta.

That region encompasses the Gulf of Mexico, which has more than 150 oil rigs and tens of thousands of abandoned wells. And in 2010, it was host to the world's worst-ever oil disaster.

Resources above the sea floor versus those below

To some, the alternative potential sources of value in the Great Southern Reef represent an irreconcilable conflict.

Local communities, fishing groups and environmentalists have been waging a battle that has seen two of the world's biggest oil companies — BP and Chevron — flee from plans to drill in the region and drop millions of sunk costs along the way.

But other companies are pursuing similar plans, with Norwegian company Equinor, formerly Statoil, planning to drill as soon as next year.

Mr Doman says when companies do drill in the area, they will do so safely.

"We don't think there's anything about the Great Australian Bight that makes it unsuitable for our activities," he says.

The Rainbow Warrrior in Amsterdam awaits departure to Newfoundland in 1981. ( Supplied: National Archive of the Netherlands )

Joining the fight to stop the oil industry's plans is the ship we are aboard: the latest version of Greenpeace's Rainbow Warrior.

The ABC was invited to spend a few days on the ship as it hosted a team of young scientists studying the reef.

The ship's voyage across the reef — and its hosting of journalists — represents an escalation in Greenpeace's campaign to try to stop oil drilling in the region.

The first Rainbow Warrior was bombed in 1985 by the French government as Greenpeace campaigned against its nuclear testing in the Pacific. Each Rainbow Warrior has been at the centre of operations against everything from whaling and illegal fishing, to coal shipping and oil drilling in the Arctic.

If nothing else, it is a powerful symbol of globalised environmental activism. It sends a signal to the oil industry — it has a serious battle on its hands.

Greenpeace's Rainbow Warrior sits in waters in the Great Australian Bight. ( ABC News: Michael Slezak )

But with drilling operations here not planned until the end of 2019 at the earliest, the ship is not protesting now.

Instead, it is helping young scientists, including Georgina Wood, conduct research they believe might help Greenpeace's campaign to stop the region being developed into an oil field.

While standing on the ship's bridge, Greenpeace Australia's head of anti-oil campaigns Nathaniel Pelle explains why his organisation is devoting so many resources to helping scientists.

"The Rainbow Warrior is on an expedition across the entire Great Australian Bight from Melbourne to Western Australia — to the Bremer Canyons — to shine a light on the truly remarkable environment," he says.

"We're here to give a platform to scientists who want to explore, who want to show the world and Australia that the reef systems down here are as important as their more famous tropical cousins."

And there's a reason why an environmental charity like Greenpeace is hopeful research it facilitates could teach the world something new and important about the region.

Despite it's great value to Australia, much of the reef system is completely unknown.

"It's extraordinary that we're about to put this ecosystem at risk when we know so little about it," Mr Pelle says.

The great unknown

Ms Wood emerges from the water after her third and final scuba dive for the day — one day in a week-long trip on the Rainbow Warrior.

She packs away her equipment and files the notes she took on her waterproof paper.

Marine biologists Stefan Andrews and Georgina Wood return from a dive in the Great Australian Bight. ( ABC News: Michael Slezak )

The area her team just examined — about 200 kilometres from Adelaide — had never before been surveyed by scientists.

And almost everything she sees in the water gets her excited.

"In the survey we found a lot of cool invertebrates, there's a huge diversity of sea stars down here, quite a lot of larger wrasse. And I was really excited because there are about four different species, or maybe even five, of seagrass that we found, which I'm particularly interested in," Ms Wood says.

Marine biologist Georgina Wood says an oil spill "could definitely tip these really sensitive ecosystems over the edge". ( ABC News: Michael Slezak )

The surveys she conducts are a standardised type that allow results from different areas to complement each other.

But the system she is examining is changing at a rapid pace.

Pressures such as urban development and water pollution are taking their toll, and climate change has been the most devastating.

In 2016, the Great Barrier Reef was dominating headlines as warm waters bleached and killed corals there.

But at the same time, entire kelp forests on the Great Southern Reef were killed and scientists said they were gone forever.

Ms Wood says the water is going to keep warming and there's nothing much anyone can do about that in the immediate term.

She says there are other threats that can be controlled — chief among those is planned oil drilling.

"If there's a spill that could definitely tip these really sensitive ecosystems over the edge," Ms Wood says.

"You can't continue to push them. Once they've been tipped too far, it's really hard to bring them back."

Just skimming the surface

It's not just the environmentalists helping scientists to survey these waters to see what drilling could put at risk.

The oil industry itself has paid for some of the most in-depth research ever conducted on the Great Southern Reef. And what it has discovered is extraordinary.

An expedition crew of scientists and documentarians prepare to dive in the Great Australian Bight. ( ABC News: Michael Slezak )

Before BP abandoned its plans to develop the region, it funded a major research project led by Hugh Macintosh from Museum Victoria.

Among the roughly 1,200 different species the researchers pulled from the water, nearly a third were completely new to science.

But what's more, among those that had been seen before, a third of those had only ever been seen in a single specimen.

"Before this project we knew almost nothing about the deep sea in the Great Australia Bight, so going into it we were expecting to find lots of new things," Dr Macintosh says.

"We found new species in almost every major group of animal we studied, including sponges, corals, crabs, shrimp, worms and sea stars."

Crew and scientists prepare for a dive in the Great Australian Bight. ( ABC News: Michael Slezak )

Among them, he has a few favourites.

"I'm quite fond of a heavily armoured shrimp called glyphocrangon, but there are also some fascinating finds like acanella, a 'bamboo coral', a minute snail that is a parasite on brittle stars and a tiny, shiny squid from the genus sepiolina."

The surveys Ms Wood and her colleagues are conducting here are in shallower water than those conducted by Dr Macintosh and do not involve sifting mud from the sea floor to find hidden species.

So the animals her team finds are more likely to have been seen before.

Despite that, Ms Wood says they are likely to find species in places scientists didn't know they existed before.

But given the state of knowledge of the area, that's not a huge surprise.

"A lot of the Great Southern Reef remains to be understood," Ms Wood says.

From left to right: Rainbow Warrior crew member, Stefan Andrews (UNSW), Samual Owen (UNSW) and Richard Robinson (photographer). ( ABC News: Michael Slezak )

One reason so little is known about the area is the difficulty people face trying to work in the water.

"We've got very harsh conditions; big swells, currents, colder waters," she says.

"It can be really, really beautiful but it can also be quite challenging and that's rewarding in itself as well."

And for Greenpeace, it's those very conditions that make them worry a disaster can occur.

The risks

Greenpeace's Mr Pelle is keen to remind people how bad an offshore oil disaster can be.

"People will remember the Deepwater Horizon oil spill in 2010. If we allow companies to drill for oil here, it's rougher, it's more dangerous and that's a real threat to the ecosystem," he says.

That Deepwater Horizon rig, owned by BP in the US Gulf of Mexico, exploded, resulting in the largest marine oil spill the world had ever seen.

BP was found responsible for the spill because of "gross negligence" and actions that were "reckless".

But it took five months for the company to contain the well and during that time millions of barrels of oil spread through the environment and devastated marine species and nearby industries.

A new underwater remote-operated vehicle takes to the water in the Great Australian Bight. ( Supplied: Richard Robinson/Greenpeace )

Mr Pelle says the response to the accident in the Gulf of Mexico would be impossible in the Great Australian Bight.

The area is undeveloped, in much deeper water and further from land, he says.

"The oil companies themselves admit these are harsh conditions. It is a kilometre deeper than Deepwater Horizon," Mr Pelle says.

Marine ecologist Sam Owen and radio operator Till Seidensticker set up their new underwater remote-operated vehicle. ( Supplied: Richard Robinson/Greenpeace )

But oil industry representative Mr Doman says that comparison isn't relevant.

"The oil and gas industry has operated in similar environments around the world," he says.

"What we see too often is that public discussion distorted by false and exaggerated claims around the impacts of our industry.

"Certainly there are risks. They have to be managed.

"But here in Australia, in Australian conditions, under Australian regulations, we have a very strong track record of safe and sustainable operations delivering significant energy supplies to this country and our export partners."

The reassurances leave Mr Pelle unmoved.

"The exact claims that the oil companies make about the Great Australian Bight are the claims they make before every accident in the oil industry's history," he says.

"Wherever there is drilling then accidents are inevitable. It's only a matter of how bad they are and where and when they are to occur."

Dr Macintosh is more on the fence about the prospect of an oil industry in the region.

"I don't know how localised any effects of drilling would be," he says.

"I hope that policy makers can look at the work we've done and factor the diversity of the Great Australian Bight into any decisions they make."

As we leave the ship and it continues to Western Australia, oil companies continue their efforts to find oil to extract from below the sea floor.

If they succeed, it is likely this won't be the last time the Rainbow Warrior will patrol the Great Australian Bight.