Around 170,000 Cambodian workers have headed home in the past week, according to the International Organization for Migration (IOM), although the exodus is now slowing. Many left after hearing rumours that Thailand's junta was bent on cracking down on illegal migrants.
The generals who seized power on May 22 to end six months of political turmoil have promised no action against those working legally in Thailand. But junta leader Prayuth Chan-ocha pledged last week to "tighten" laws applied to foreign labourers.
"I admit there must be some impact on business, but I don't know to what extent," Sihasak Phuanketkeow, the foreign ministry's permanent secretary, told reporters after assuring Cambodia's ambassador that the military planned no crackdown.
The junta blames the departures on "unfounded rumours" of imminent action against illegal workers. Tanit Numnoi, a senior Ministry of Labour official, said workers could return once their papers were in order.
But Cambodians heading down the potholed roads to the border in packed buses and trucks were having none of it.
Kiew Thi, 38, said it had taken him hours to reach the checkpoint.
"I'm going back because I'm afraid soldiers are going to come and get us," Kiew Thi said. Like others, he had been drawn to a job in the Thai fishing industry by monthly wages of 8,000 baht ($250), considerably more than he could earn at home.
The Thai economy, Southeast Asia's second-largest, is heavily dependent on migrant workers, mostly from Myanmar, Laos and Cambodia. Migrants cross porous borders to perform jobs most Thais are unwilling to do in labour-intensive sectors.
"This will definitely impact the construction industry, particularly along the eastern seaboard of Thailand, a key economic region. It will also affect agriculture as some fruit orchards rely on Cambodian workers," Vallop Vitanakorn, Vice- Chairman of the Federation of Thai Industries, told Reuters.
"But I believe once they have their work documents in order most of them will return, perhaps within a month or two."
MORE THAN 2 MILLION MIGRANT WORKERS
The labour ministry says there are more than 2 million legally registered foreign workers in Thailand. More than half come from neighbouring Myanmar, formerly known as Burma.
But Burmese labourers have not joined in the rush to the border and rights groups told Reuters they were trying to allay any fears of impending deportation.
A national verification programme requires migrants to secure passports at home in order to apply for, or renew, Thai work permits. Thai officials had previously turned a blind eye to many provisions of employment laws.
Military authorities now propose policies with nationalist overtones, including the creation of economic zones for migrant workers in border areas in order to free up more jobs for Thais.
Sihasak, the foreign affairs ministry's top official, said there could be a positive spin-off from the past week's departures.
"This will be a good thing for the country because we can put in order the workforce and make it legal," he said. "We don't want foreign workers to be exploited by their employers."
The flow of migrants heading home had eased somewhat over the past 24 hours, according to Brett Dickson, IOM's team leader in the Cambodian border town of Poi Pet.
"There are a lot of Cambodian military trucks picking people up and people are getting out of here within a couple of hours," he said. "The next challenge is helping those who want to return to Thailand in the next few months get proper work documents in order." ($1 = 32.3500 Thai baht).