Robyn and Stuart Mackenzie, riding motorbikes one day in 2006 on their vast sheep and cattle farm in the Australian outback, spotted a pile of what looked like large black rocks.

On close inspection, they appeared to be dinosaur bones. An even closer inspection, with the help of paleontologists who were part of a new study, found that they belonged to a new species of dinosaur that is the largest ever found in Australia and one of the largest in the world.

Researchers in Eromanga, Queensland, where the Mackenzies live, said on Monday that they had identified the new species, calling it Australotitan cooperensis. Nicknamed Cooper after a creek near the fossil site, it was a long-necked, plant-eating titanosaur estimated to have lived more than 90 million years ago, during the Cretaceous period. Like the brachiosaurus, the titanosaur was part of a group called sauropods, which were the largest of all the dinosaurs.

Though closely related to three other titanosaur species discovered in Australia, Australotitan was significantly larger. It is estimated to have weighed about 70 tons, measured two stories tall and extended to about the length of a basketball court, making it comparable in size to the gargantuan titanosaurs that have been found in South America. The researchers’ findings were published on Monday in the journal PeerJ.

“This is sort of our first hat in the ring, getting into the big leagues of big titanosaur dinosaurs worldwide,” said Scott Hocknull, a paleontologist at the Queensland Museum in Brisbane and a co-author of the study. “We’re pretty excited about it because it’s just the start of what we think might be a new wave of discoveries of very large dinosaur species in Australia.”

Unlike in the United States, where dinosaur fossils have been hunted since the mid-19th century, Australia began its “dinosaur rush” relatively recently, with a flurry of discoveries in the past two decades. Like the three other titanosaur species, Australotitan was found in the Winton Formation, a thick layer of sedimentary rock that covers large parts of the state of Queensland.

“It’s a very exciting discovery and analyzed in a fantastic way,” said Matt Lamanna, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh who has studied multiple titanosaurs.

Dr. Lamanna said the study, which he was not involved in, showed that during the Cretaceous period “Australian dinosaurs were getting as big as the biggest dinosaurs that we know about.”

He also praised the 3-D visualizations of Australotitan’s bones, which are so heavy they can require several people or even a forklift to move. The visualizations make it easier to compare them with bones from other species and “set a new standard for the analysis of a giant titanosaur like this,” Dr. Lamanna said.

Because it is mostly flat and lacks the mountain ranges and canyons that would expose fossils by eroding rock, Australia is one of the most difficult places to find dinosaurs in the world. Rather than people finding dinosaurs, Dr. Hocknull said, “dinosaurs find you.”

For that reason, farmers have proved important, because they spot fragments of dinosaur bone that have migrated to the surface of their land. There could be “beautifully preserved” bones in the black soil several feet below the surface, Dr. Hocknull said, but major machinery is required to excavate them. Local communities in Queensland like Eromanga, a town of about 60 people (that is occasionally misplaced by Google Maps), have played a key role in recovering fossils.

A dinosaur bone was first found on the Mackenzies’ property in 2004 by their son, Sandy, who was 14 at the time. Early on, Ms. Mackenzie said, she and her husband decided it was best for the bones to stay in Eromanga, “rather than being sent thousands of kilometers away to a state museum.” So they started a museum themselves.

Neighbors rallied to help. A local earth-moving business provided the machinery for excavation, an oil refinery supplied fuel, and others in town offered funding or were trained as volunteers.

“We physically couldn’t have gotten the bones out of the ground without the community supporting us in the way that they did,” said Mr. Mackenzie, who is also mayor of Quilpie Shire, an area that includes Eromanga.

Excavation began in 2007, followed by years of preparation and analysis using 3-D scanning to compare the bones with those from existing species.

The Eromanga Natural History Museum opened in 2016, with a similar mission as a natural history museum in the town of Winton run by a nonprofit group called the Australian Age of Dinosaurs. The establishment of such museums is “probably one of the largest citizen-science campaigns in Australia, certainly for natural history,” Dr. Hocknull said.

“Without them, none of these fossils would be found, none of them would be excavated, none of them would be prepared and none of them would be scientifically available,” he said.

The Mackenzies say significant excavations continue on their property, including one with bones that may also prove to be from a new species.

“I will never see all this material processed in my lifetime, there’s just so much of it,” said Ms. Mackenzie, who is now a field paleontologist, head of the museum and, along with her husband, a co-author of the new study. “So we need this sort of facility out here to keep employing people.”

The museum is changing the way that people in Eromanga think about the town’s future. With pandemic border closures having kept Australians fenced in at home for more than a year, domestic tourism has become much more popular, Mr. Mackenzie said. He hoped the dinosaur discoveries in Eromanga would also draw more international visitors once the country reopens, he said.

Dr. Hocknull said the museum also gave the children of Eromanga a firsthand science education that they might otherwise not have access to.

“They’ve probably never been to Brisbane, they’ve never seen the ocean, and yet here they have Australia’s largest dinosaur literally in their backyard,” he said.