When floodwaters hit Bangkok, migrant worker Tha Gay was immediately fired. He lost his work permit and with it his legal status, then was arrested and deported back to Myanmar.

But the 23-year-old construction worker was soon back in Bangkok after he borrowed 16,000 baht ($500) and paid a broker for a space in an overloaded pickup truck to the Thai capital, where he's now broke, unemployed and illegal.

"I lost everything in the floods -- my job, my papers, my housing," he told AFP at a safe house in the city's still-flooded Lad Phrao district.

"I'm now in debt from the trip back too. I've been looking for work but as I have no papers, no one will take me so far. I've got nothing."

The story of Tha Gay, who was detained for eight days before he was sent home, is not unusual.

He is among about 100,000 migrants from impoverished Myanmar -- of a migrant workforce of two million, legal and illegal -- who returned home after the devastating floods hit Thailand this summer. About half went voluntarily.

Activists warn that as they seek to make their way back over the border and head to Bangkok, they are turning to old smuggling routes, leaving them vulnerable to trafficking and exploitation.

A Myanmar aid worker based in the Thai capital who asked not to be named said that it was likely there would be an increase in trafficking after the floods.

"Traffickers and brokers were very happy with the floods -- it was a real bonanza for them," she said.

Some workers, having lost their jobs and housing to the floods, fled out of choice and faced extortion and abuse by immigration officials and police along the way, activists and experts say.

Others fell foul of Thai regulations which ban registered migrants from leaving their province of employment -- even if they are escaping chest-deep flood waters -- and were unceremoniously deported.

Those who stayed on in flooded areas struggled to get by as they were largely excluded from mainstream relief efforts, the United Nations said last week, adding it was "concerned" about the lack of support for migrants.

Thailand's economy relies on migrant workers from its poorer neighbours Myanmar, Cambodia and Laos.

When unusually heavy monsoon rains caused a deluge that swept across much of central and northern Thailand from July -- leaving more than 600 people dead and damaging millions of homes -- migrant communities were hit hard.

"Abuse of migrant workers in Thailand is well documented, but the floods magnify this to a new level," said Sunai Phasuk, a Thailand researcher for Human Rights Watch.

Very little has been done by the Thai government and authorities to help migrant workers during the crisis, Sunai said, despite the fact that their labour will be essential after the waters recede.

"We're talking about a massive reconstruction -- but I don't think it will get better (for migrants) as billions of dollars pours into reconstruction, not judging by the government's action during the flooding," he said.

The government has said its priority is to help factories recover and get back to work, not bring in new workers.

"So far, we don't have any information that businesses lack workers. In my opinion, I don't think there will be any problem of labour shortage nor that it will affect the recovery of industrial estates or reconstruction," labour minister Phadermchai Sasomsab told AFP.

For deported migrants who are trying to get back -- or new migrants hoping for work during the reconstruction -- "the only route is smuggling", said Andy Hall, a migration policy expert at Mahidol University in Bangkok.

Tha Gay, whose name AFP has changed for his safety, said his broker ensured he at least made it back to Bangkok, but added his journey was "terrible" and had convinced him never to risk it again.

"It was the most terrible journey, there were 12 of us crammed in the back of the pickup, people moaning, crying and vomiting. I just wanted to die," he said.

"If they deported me now, I wouldn't come back again -- the problem is, I need to pay back my debt. I just hope I can find work."