A deadline for undocumented migrant workers in Thailand to register for temporary permits passed on Saturday without a major exodus of workers to Cambodia, NGOs say, although observers were still unsure whether a crackdown on remaining workers is in the offing.

Barry Jessen, a senior manager at NGO Samaritan's Purse, which works on the Thai-Cambodian border, said he had noticed no uptick in workers crossing back into Cambodia.

According to Jessen, the vast majority of migrants who returned said they were travelling for Khmer New Year. "None of them were saying 'I'm coming home because of the pink card deadline,'" Jessen said. "So far it doesn't seem like there's been a massive crackdown."

Workers who registered for their temporary "pink cards" by the the March 31 deadline now have until June to finish the regularisation process.

Those that missed the deadline will have to come back to Cambodia and re-enter through a legal channel. It was unclear how many workers missed the deadline, although a report in the Bangkok Post put it at roughly 60,000.

Reuben Lim, a spokesman for International Organization for Migration in Thailand, said the body was unsure whether the Thai government would move to crack down on undocumented migrants still in the country.

Lim noted that April typically sees large amounts of cross-border movement between Cambodia and Thailand, with the Khmer New Year, or Songkran, festival celebrated in both countries.

Last year, Lim said, the number of migrants returning in April was higher than in July, when the Thai government announced its crackdown.

"While it would not be surprising to observe large movements in the coming days or weeks, it will be difficult to attribute this solely to the March 31 deadline as Cambodians may also be returning for the Songkran holidays," Lim said.

On Friday, the Ministry of Labour announced that it struck an agreement with the Thai government to allow Cambodian migrant workers to travel between the two countries from April 5 to April 30 without re-applying for visas, as long as they leave and return through the same border checkpoints.

Updates to follow