Shutting Borders Helped Australia Contain Covid-19. Now Reopening Them Is Proving Difficult.

Passengers were told they would either have to pay for a return flight to South Australia or commit to a 14-day quarantine on arrival in the city of Perth.

Health officials and lawmakers in Australia say they are convinced that the country’s success in suppressing the coronavirus owes much to a series of border controls that are among the strictest in the Western world, although it has come at a cost to businesses.

Now, as Australia and other governments prepare to roll out vaccines and reopen their economies, deciding when to let people travel freely is proving difficult.

Australia’s controls not only limit international arrivals to Australian citizens and permanent residents, or New Zealand residents living in Australia, but in many cases they also restrict people’s ability to move around the country.

Australia has recorded about 28,000 coronavirus cases, mostly among quarantined travelers from overseas, compared with more than 15 million cases in the U.S. Still, pandemic restrictions tipped Australia into its first recession in 29 years, illustrating the trade-off between the economy and health.

Vaccine distribution is likely to be uneven, targeting high-risk groups first. Some countries will find it easier to distribute vaccines in areas with clinics and other health infrastructure than in remote regions. Unfettered travel early in the vaccine rollout risks causing new outbreaks if people think the pandemic is beaten.

Australia is starting to roll back restrictions, the culmination of monthslong and often fractious talks between leaders of Australia’s states and territories, who are responsible for internal borders. Many of those leaders have found the hard-line approach to borders popular with voters, even as businesses complained of rising costs.

Australians have typically needed to quarantine for 14 days when they cross state lines, sometimes in government-supervised hotels. Facial-recognition technology via an app, used in conjunction with cellphone-location data, has been deployed to strengthen compliance of people isolating at home. Some states have even refused to accept residents returning from regions with higher infection rates, leaving them stranded for months.

In the U.S., a patchwork of restrictions exists that is mostly aimed at people coming from hot spots. While the governors of California, Oregon and Washington ask visitors from most other states to self-isolate for 14 days, lengthy quarantine requests are typically the exception. Most Hawaiian islands let travelers from the mainland skip quarantine if they show proof of a recent negative Covid-19 test.

Many U.S. officials attribute rising cases to complacency in travel. The Centers for Disease Control and Prevention urged people to stay home for the Thanksgiving holidays, cautioning that travel increased the chance of getting and spreading Covid-19.

In Australia, quarantine requirements are enforced with the threat of fines or jail time.

“Clearly it was one of the major strategies we had,” said Adrian Esterman, chair of biostatistics and epidemiology at the University of South Australia. And “not only did we suppress Covid-19, we crushed it.”

Take Western Australia, a state so dependent on mining that it produces more than half of the world’s iron ore traded by sea. A hard border with the rest of Australia challenged its biggest miners, which typically fly in skilled labor such as engineers from other states. Mining entrepreneur and former lawmaker Clive Palmer took Western Australia’s government to court over its border policy, arguing it breached the constitution. He lost.

A recent poll by JWS Research found 87% of Western Australia residents rated the state government’s response to the pandemic as good or better, the highest rating in the country. Western Australia has recorded roughly 830 coronavirus cases.

Western Australia resident Rodney Kirk-Burnnand and his wife, Pauline Allen, began a three-month vacation on the country’s east coast in February. Eight months later, they were still stuck there, unable to get permission from state authorities to return home.

Authorities eventually allowed the couple to re-enter Western Australia on the condition they made the 900-mile drive only stopping for gas. At 74 years old, Mr. Kirk-Burnnand complained the requirement put him more at risk of a road accident than coronavirus. When he got home, neighbors called the police to report him, he said.

Genomic testing has helped to show how easily the virus jumps state lines.

Roughly 900 of Australia’s cases were linked to passengers who had been allowed to leave a cruise ship that docked in Sydney in March. A second cluster in southwestern Sydney in June was traced to a man who had traveled from Melbourne, where a virus outbreak was worsening.

Those incidents make many people nervous about relaxing restrictions too soon. About two-thirds of Australians worry that open borders make states vulnerable to outbreaks elsewhere in the country, the JWS Research survey found. More adults feel borders should close to states with active cases than believe they should never close.

The economic and social costs are mounting. Domestic air travel in November was at 31% of year-earlier levels, according to Cirium, an aviation analytics company. In contrast, seat volumes in the U.S. had rebounded to 60% and in Japan to 81%, while China had surpassed year-ago levels.

Passenger numbers between Sydney and Melbourne, historically the world’s second-busiest air route, plunged more than 90% year-over-year from March to October, according to an EY report.

The Business Council of Australia, an industry group, and consulting firm EY estimate the cost of grounded domestic flights at 2.1 billion Australian dollars, or about $1.6 billion, a month.

Students at boarding schools haven’t been able to go home to visit parents during vacation breaks. Some health-care providers have been unable to get to work, and some patients unable to travel to their usual doctors. Debate raged about whether state lines were moved to alleviate traffic at checkpoints in border towns, where residents needed to cross back and forth for work or other reasons.

Prof. Esterman’s daughter is in Melbourne, across the border from South Australia. He hopes she can return home for Christmas.

“How many other people in Australia are like that?” he said. “An awful lot.”

—Rachel Pannett contributed to this article.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com