The Indonesian and Malaysian markets are being excluded from the approval list to visit the Phuket sandbox, though there is demand to visit Thailand in those countries, says the Tourism Authority of Thailand (TAT).
Sophon Tantayotai, director of the TAT's Jakarta office, said Indonesian tourists can take overseas trips and travel sentiment on social media remains positive.
However, tourists have to undergo 14-day quarantine because Indonesia is listed as a high-risk country following an outbreak in July, which peaked at 60,000 cases per day.
Indonesian authorities report roughly 20% of the population or 46 million people are fully vaccinated, though Jakarta is near 100%, with the situation improved since mid-August as daily infections dropped to 3,000-5,000.
He said Indonesian travellers also face an eight-day quarantine when returning, which is another obstacle for visitors who on average spent 5-7 days in Thailand before the pandemic.
"The Phuket sandbox alone might not attract huge demand as tourists would like to combine Bangkok in their plans. Reopening the capital will help revive volume from the Indonesian market," Mr Sophon said.
However, foreigners are not allowed to visit Indonesia for leisure, thus international flights remain grounded.
He said if international flights can restart and travel restrictions are lifted, the Indonesian market will rebound strongly by next February, as there were 20,280 seats available per month on flights to Thailand pre-Covid.
In 2019, there were 709,613 travellers from Indonesia to Thailand generating 18.2 billion baht.
Malaysian tourists are not allowed to take leisure trips abroad and the country is excluded from the Phuket sandbox, said Nongyao Jirundorn, director of the TAT's Kuala Lumpur office.
Prior to the outbreak, Malaysia was the second largest market for Thai tourism with 4.2 million tourists generating 107 billion baht, as Thailand is the most popular outbound destination.
Some 60% came by land, with the most popular destinations Songkhla, where the Sadao border checkpoint is located, followed by Bangkok, Krabi, Phuket and Chiang Mai.
She said tourists are interested in the sandbox programme, however the Malaysian government would like to promote domestic trips in Malacca, Langkawi or Sarawak as 79.6% of the population of 32 million were fully inoculated as of Sept 20.