SYDNEY—Elle Robertson spent much of this year living on her parents’ farm in the Australian countryside instead of going to her university courses in London.

But Ms. Robertson is now on her way back to England, on board one of the first international flights to leave Sydney since Australia dropped one of the world’s strictest coronavirus travel bans that had been in place for nearly 600 days. Starting Monday, Australia reopened its border for citizens and permanent residents, who are now allowed to travel freely into and out...

SYDNEY—Elle Robertson spent much of this year living on her parents’ farm in the Australian countryside instead of going to her university courses in London.

But Ms. Robertson is now on her way back to England, on board one of the first international flights to leave Sydney since Australia dropped one of the world’s strictest coronavirus travel bans that had been in place for nearly 600 days. Starting Monday, Australia reopened its border for citizens and permanent residents, who are now allowed to travel freely into and out of the country if they are vaccinated.

“It’s just felt like I put my life on pause,” said Ms. Robertson, 21 years old, shortly after checking into her flight. “Every time you think Covid’s getting better, it’s like, oh no, everything’s shut again. Hopefully this time is for good,” she said.

Some of the world’s last travel holdouts, which until recently were intent on keeping borders closed in an effort to eliminate the spread of the coronavirus, are now finally opening up as Covid-19 vaccination rates rise.

Elle Robertson, wearing a backpack, with her parents, Bruce and Fiona.

Photo: Mike Cherney

Thailand on Monday began allowing vaccinated tourists from more than 60 countries to enter without requiring extended hotel quarantines. New Zealand also recently said it would add more Pacific nations to its quarantine-free travel zone and halve the time citizens who travel from other countries need to spend in government-run quarantine.

These countries have typically experienced low rates of infections by global standards. New Zealand, which has a population of about 5 million, has recorded about 6,600 Covid-19 cases since the pandemic began while Australia has reported 172,000 total cases among its 26 million population, according to Johns Hopkins University.

In the U.S., the Biden administration plans to lift travel restrictions for international travelers in the coming days. The new policy will affect both air and land-border travelers, who will need to be fully vaccinated. The U.S. has 58% of its 330 million people fully vaccinated and has recorded nearly 46 million cases.

Even as more countries drop travel restrictions, it could be years before international tourism reaches pre-pandemic levels. Almost half of all experts recently polled by the United Nations World Tourism Organization see international tourism returning to 2019 levels in 2024 or later, while 43% point to a recovery in 2023. The agency said 54 million tourists crossed international borders in July 2021; though this was down 67% from the same month in 2019, it was the strongest result since April 2020.

Tourism is most challenged in Asia and the Pacific, where many countries, including Australia and China, have had success in controlling the coronavirus by closing their international borders and have been slow to reopen due to fear of seeding new outbreaks. In the January to July period, international arrivals in Asia and the Pacific fell 95% compared with 2019, versus an 80% decline for the world at large, according to the U.N. tourism agency.

Many of those countries are now dropping their so-called Covid-zero strategies as vaccination rates rise and the Delta variant of Covid-19 proves too infectious to fully eliminate; they are instead pivoting to learning to live with the virus.

The arrival area at Phuket International Airport in Thailand. The country’s new rules require vaccinated tourists to spend one night in a hotel while they await a Covid-19 test result.

Photo: mladen antonov/Agence France-Presse/Getty Images

China is the largest exception: The world’s most populous nation has committed to maintaining “zero tolerance” for the virus despite criticism from business groups and a close to 80% vaccination rate. On Sunday, Shanghai’s Disneyland was temporarily shut down after a visitor who attended the day before was found to be Covid-19 positive, and nearly 34,000 people were required to be tested before leaving the resort.

Thailand’s new rules require visitors from the designated countries to spend one night in an approved hotel while they await a Covid-19 test result, after which they can move freely. Some countries that are allowed quarantine-free entry to Thailand still require quarantine on return, however, which may temper the resumption of travel.

Thailand began experimenting with new travel policies earlier in the year, when it launched a program called the “Phuket Sandbox” allowing quarantine-free entry for vaccinated travelers. Those who arrived in Phuket, a resort island in the country’s south, were required to spend at least 14 days there before being allowed free movement throughout the country. The program was later expanded to several other provinces.

Thailand, with a population of about 70 million, has fully vaccinated 42% of the population and recorded 1.9 million cases, according to Johns Hopkins University.

In Australia, Prime Minister Scott Morrison has tied his country’s border reopening to the country’s vaccine rollout, which has gained momentum in recent months despite a slower-than-expected pace earlier in the year because of supply issues. Around 65% of Australia’s population was fully vaccinated by the end of October.

The country had earlier experimented with a travel bubble with New Zealand, but it had mixed success with flight schedules frequently disrupted by local Covid-19 outbreaks.

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“The ground staff, the ground crew that is really getting Australia to take off today is the Australian people, who have gone out there and kept their part of the deal and have been getting vaccinated,” Mr. Morrison said at a recent press conference.

Before Monday, Australians typically weren’t allowed to leave their country without getting an exemption, which often required a family member to be critically ill. Those seeking to return to Australia didn’t need permission to do so, but many expats complained it was difficult to find seats on commercial and government-financed repatriation flights because there was limited space in the country’s hotel quarantine system.

All returned travelers previously needed to quarantine at government-run hotels or camps. Starting Monday, returned travelers who are fully vaccinated and going to certain Australian cities, such as Sydney and Melbourne, don’t need to quarantine, although some states where there has been little community transmission of the virus, including Western Australia, remain closed.

Skilled migrants and tourists still aren’t allowed back into Australia, though officials have said that could happen in the coming weeks or months.

Lisa Poole, an Australian citizen who is originally from the U.K., was flying back to the U.K. on Monday to be the maid of honor at a longtime friend’s wedding. Ms. Poole, a dental nurse, said the wedding had already been postponed twice before and couldn’t be postponed again.

“I sort of got used to the idea it wasn’t going to happen,” she said of the trip. “It would have been a sad day.”

Ms. Robertson, the student, said she was able to attend some classes online, but that sometimes required her to stay up until 3 a.m. because of the time difference. She said she applied for a travel exemption multiple times, but kept getting rejected.

Her parents, Bruce and Fiona Robertson, came to see her off.

“Youth of today can get on with their lives,” Mr. Robertson said. “That’s the important thing about this whole opening up.”

—Feliz Solomon contributed to this article.

Write to Mike Cherney at mike.cherney@wsj.com