In a statement Tuesday afternoon, the BI said Commissioner Jaime Morente approved Fox's release "after it was established that the Australian nun holds a valid missionary visa and, thus she is a properly documented alien".
The missionary visa of Fox was issued on October 15 last year and will expire on September 9 this year.
Reports said the Australian nun was taken by two teams of BI agents who went into her house in Project 2, Quezon City.
Sister Patricia Fox, regional coordinator for the Sisters of Our Lady of Sion, was taken to the immigration bureau in Manila and is expected to be deported from the country.
The bureau said the basis of the arrest order was a provision of the Philippine Immigration Law of 1940, which says "any alien who remains in the Philippines in violation of any limitation or condition under which he was admitted as a nonimmigrant" can be arrested or deported by the bureau.
The Catholic Bishops' Conference of the Philippines (CBCP) News reported that Fox has worked in the Philippines for 27 years helping farmers and indigenous peoples (IPs).
Sister Fox had been involved with human rights missions in Mindanao in the country's troubled southern region, looking into the plight of indigenous people and farmers, leftwing lawmakers said, nearly a year after President Rodrigo Duterte imposed martial law on the southern island of Mindanao. "She extended help to the Hacienda Luisita, Negros and Mindanao farm workers on their land cultivation campaign", they said.
Almost a day after she was apprehended, the Bureau of Immigration (BI) said it granted the release of 71-year-old Australian Catholic nun Sister Patricia Fox on Tuesday, April 17.
In a statement, the seven-man Makabayan bloc laments that the nun's arrest was part of the Duterte adminitration's crackdown against critics, the opposition and even human rights advocates.
Rights groups criticised Fox's detention as a warrantless arrest.
Mr Pahilga says Sister Fox was not involved in any rallies during her mission work.
"This shows the global community that the democratic space in the Philippines is shrinking. People should realise the government has a tendency of being dictatorial", Pabillo said.
Philippines' Intelligence services (NICA) said they had a photos of her raising a clenched fist with the banner "Stop Killing Farmers" as well as wearing a Philippines agricultural union T-shirt.
"Contrary to the claim of NICA, Sister Pat did not join any rally in Tagum or Davao or in any province of Region XI for that matter".
Reuters was unable to contact Sister Fox for comment.
It's unclear how long this will take. He said the special prosecutor was inclined to release her immediately but she did not have her passport with her when she was arrested.
"There seems to be a government war on foreigners who have shown solidarity with Filipinos". She allegedly told them she was forced to sleep on the floor of the BI security office during her time under arrest. This is already happening without martial law.