Four women were detained at Manila's international airport on New Year's Day while about to depart for Phnom Penh, indicating that surrogacy clinics are still operating in the city despite a crackdown on commercial surrogacy there.

Philippine authorities have also been put on alert for another group of surrogates who have been hired to travel to Phnom Penh on an undisclosed date, officials said.

During questioning the detained women said they were promised about US$10,000 ($13,800) to be impregnated with the sperm of intending fathers from Australia, Germany, China and Nigeria, officials said.

Philippine Immigration Commissioner Jaime Morente said the detentions revealed "a new modus operandi of a human trafficking syndicate that preys on our Filipino women who are enticed to bear children of strangers for a fee because of their poverty".

"We cannot allow this to happen," he said in a statement.

It is unclear whether the alleged traffickers were planning for the mothers to give birth in Cambodia or another country.

Cambodian authorities declared in October that commercial surrogacy constitutes human trafficking pending the passing of a law which has yet to be drafted.

They arrested Australian surrogacy broker Tammy Davis-Charles whose company Fertility Solutions PGD allegedly signed at least 25 surrogacy agreements, most of them with Australian biological parents who paid US$50,000 per baby.

A multimillion dollar surrogacy industry emerged in Phnom Penh in 2015 after surrogacy operators were chased out of Thailand, Sri Lanka and India.

The crackdown in Phnom Penh prompted scores of pregnant surrogates, most of them from impoverished villages, to go into hiding, fearing arrest, or to travel to countries like Thailand, where the Australian embassy in Bangkok is still processing passports for surrogate-born babies.

In 2014, Thailand's military government shutdown a booming surrogacy industry in Bangkok following the Baby Gammy scandal.

More than 70 Cambodian women are believed to be pregnant with the babies of Australian biological parents.

Cambodia's Secretary of State at the Interior Ministry, Chou Bun Eng, has urged the Australians involved to declare they are the biological parents or risk being treated as suspects in human trafficking.

Ms Bun Eng has also urged the surrogates to come forward so they can receive medical checks and medicines during their pregnancies.

The Filipino connection illustrates the flexibility of international surrogacy agents who operate across multiple borders, flying surrogates, eggs, doctors and parents to whichever country is the most porous for their business.

The operators look for poor, lightly regulated countries that don't have laws dealing directly with surrogacy, such as Cambodia.

There are no laws governing surrogacy in the Philippines where, in the first known surrogacy case in 2008, a baby boy was born in an arrangement between a gay male couple from Malaysia and Denmark and a married Filipino woman.

At least one surrogacy agency in Manila advertises services for international clients.

Any attempt to legalise commercial surrogacy would likely be opposed by the Catholic Church in majority-Catholic Philippines.

Fairfax Media has previously revealed that many of the 50 surrogacy operators who set-up business in Phnom Penh in 2015 and 2016 have now moved to neighbouring Laos.

Australia's smartraveller.gov.au website warns that commercial surrogacy is illegal in Cambodia, Laos, Thailand, Nepal and India.

For months, the Turnbull Government has been sitting on a parliamentary committee report recommending sweeping changes to Australia's handling of surrogacy overseas, including that arrangements Australians enter into be the subject of detailed scrutiny to protect the rights of both birth mother and child.

The report recommended that commercial surrogacy remain illegal in Australia but supported allowing "altruistic surrogacy", where costs such as legal and medical expenses are reimbursed to surrogate mothers.

It acknowledged that laws in Queensland, NSW and the ACT banning Australians from seeking surrogacy abroad are ineffective, after thousands of Australians have gone overseas for such services in recent years.