The tourism operators who depend on the natural wonder, already contending with climate change, have been walloped by the pandemic. The absence of visitors is bad for them — and the reef.

Diving beneath the ocean, Russell Hosp swam toward the limestone bed of the Great Barrier Reef, where he reattached bits of blue staghorn coral. With tourists gone, he was filling the void with this small act of conservation, which took his mind off the uncertain future on land.

"It was a bit surreal," Mr. Hosp, a reef guide, said of spending hours at sea unaccompanied by the usual enthusiastic visitors. Aboard the quiet catamaran, he said, he realized just how much the coronavirus "had changed the world."

The pandemic has fast-forwarded a looming reckoning for the tropical city of Cairns, the main gateway to the reef and the base for Mr. Hosp and many others whose livelihoods depend on it.

Tour operators there were already fighting a perception that the reef is in its death throes, as warming waters cause repeated mass bleaching that has robbed many corals of their vivid colors. But where climate change has been more of a creeping threat to the reef’s survival, and thus to Cairns’s tourism lifeblood, the coronavirus has delivered a hammer blow.

Now this city, so linked with the natural wonder just off its shore that it can scarcely imagine life without the visitors who come in droves, has been forced to confront the prospect that it can no longer depend on tourists.

Foreign and local travelers, already deterred by last summer’s devastating bush fires and now locked out by Australia’s international and domestic travel bans, have all but vanished, and a $4.6 billion industry built around the world’s largest living structure has ground to a near halt.

The sudden disappearance of visitors feels all the more unreal because the virus itself has barely touched Cairns: The city of 150,000 people in far northeastern Australia has recorded only a couple of dozen cases, and has none currently.

But there is no escaping the reach of the pandemic.

"We’d never stopped running before — the global financial crisis, terrorism attacks, airline strikes; you name it, the world has thrown it at us," Mr. Hosp said. "We don’t know if we’ll ever get back to normal."

In Cairns, visitors who usually cram the jetty every morning as they wait to pile onto boats have dwindled from the thousands to a few hundred, leaving operators out of work, boats moored at the dock, and some hotels and restaurants shuttered.

Storefronts on the main drag are for lease, and the esplanade, usually heaving with tourists at dusk, looks like something out of a sleepy beach town.

"It’s been so quiet," said Heather Forbes, a Cairns resident, adding that because the city had been dependent on tourism for so long, it was difficult to know how to diversify its economy.

"I don’t think anywhere should be solely dependent on one thing," she said.

It might seem that there was a silver lining in all this, that the exodus of tourists would be a boon for the health of a reef in critical condition.

But while the abrupt absence of visiting crowds has had surprising effects in other places — monkeys overrunning a city in Thailand, deer wandering cities in Japan looking for food — the environmental impact of tourism on the reef is negligible, scientists say, especially when compared with climate change.

The reduction in international travel, and therefore planet-warming emissions, has created only a short-term benefit. The "infrastructure of fossil fuels wasn’t affected," said Prof. Terry Hughes, a global expert on coral reefs at James Cook University in Townsville, Australia.

In the end, a prolonged downturn in visits to the reef could actually be detrimental to its well-being. "Tourism provides a social and economic rationale for why the reef needs to be better protected," Professor Hughes said.

The situation has prompted the Cairns region to look more critically at its dependence on international travelers, especially those from China, who make up a large portion of reef visitors. China and Australia are engaged in an increasingly bitter diplomatic tug of war that could keep Chinese travelers away even after the borders are reopened.

"We’re realizing that we can’t rely on China," said Samantha Davidson, a travel consultant at the Reef Info Visitor Center. "It’s good," she added, because it’s sending a message to those closer to home: "Hey, come and see us."

As recent flare-ups of the coronavirus have closed state borders within Australia, some people have taken the opportunity to explore their own (very large) backyards.

"We were supposed to be in Hawaii, but we said we still wanted to take a trip somewhere warm," said Alicia Dean as she lounged in a sarong on the deck of a boat heading out to the reef. She had traveled within the state of Queensland from Brisbane, the capital, to Cairns, more than 1,000 miles to the north.

And some foreigners, stranded in Australia, figured they may as well take the time to experience the reef, a World Heritage Site. "My flight keeps getting canceled," Julia Pape, a 27-year-old from Germany, said as she donned her flippers and wet suit, ready to plunge into the tropical waters.

Tourists like Ms. Dean and Ms. Pape, however, don’t make up for the hundreds of thousands of missing international arrivals in the region, the throngs who help support the jobs of more than 60,000 people (more than those employed by Australia’s oil and gas industry). Experts have warned that even with a vaccine, it may be years before travel returns to pre-coronavirus levels.

But while the idled boats and empty storefronts tell the story of a city shaken by Australia’s travel bans — which led overseas arrivals to collapse by 99.5 percent in May compared with the previous year — in other pockets of Cairns there is a sense of relief at having made a lucky escape from the threat of infection.

Patrons at bars flout the rules of social distancing, and backpackers from overseas — many of whom have decided to ride out the pandemic wave in Australia — share close quarters in dormitories at hostels. Real estate agents say the area has drawn some clients looking to flee the danger of Covid-19.

"It’s a good place to be stuck," said Brent Bundy, a cyclist from Oregon who has been in Australia two months longer than planned. He is "in no hurry" to return to the United States, given the country’s huge number of infections, he added.

At night, parts of the city could almost be mistaken for a pre-pandemic world.

On a recent Saturday, locals and backpackers spilled out into the streets, shouting from the back of cycle rickshaws and lining up to get into the only nightclub in town. Inside, guests, mandated to sit down, had organized their stools in a circle on the dance floor, arms draped around one another’s backs. Members of the group took turns sneaking in an illicit dance.

Even this limited revelry was not possible a few months ago, with Queensland in lockdown as virus cases were rising.

With little else to do during those eight weeks, Mr. Hosp, the reef guide, and crew members from other tour companies, undertook work that the government had deemed essential: replanting hundreds of pieces of coral as part of a study on the impact of heat stress on their growth.

Under the ocean, among the parrotfish and green turtles, Mr. Hosp said, "you could almost forget what was going on in the world." But aboard the boat, the harsh reality of the virus’s impact came flooding back.

"I definitely missed the tourists," Mr. Hosp said. "It was very humbling."