Fairfax Media understands at least three men with serious sex crime convictions have been bashed by other inmates since being released into the general detainee population, rather than being housed in a segregation unit.

Christmas Island, which was formerly used to house asylum seekers, has been converted into a detention centre for about 200 foreign nationals whose visas have been revoked on character grounds under section 501 of the Migration Act.

The trouble started shortly after the sex offenders were transferred to the island late last week on a charter flight from Perth. The foreign nationals, who have had their Australian visas revoked over sex crime convictions, had previously been in protective custody units at mainland detention centres.

"It didn't take long for people to start searching their names on the internet and found out some of them had raped children," a source said. "The boys here aren't going to tolerate living with scum like that."

The unrest led centre operator Serco to put the main cell block in lockdown to try to curb the violent outbreak.

A spokesman for the Australian Border Force denied the Christmas Island facility was in lockdown.

"Careful consideration is given to the placement of individuals within immigration detention facilities...[which] includes conducting risk assessments to ensure that individuals are accommodated in a way that meets their individual needs, the individual's security risk profile and other operational considerations," he said.

Fairfax Media understands problems in the high-security "Red Block" wing has also led Serco to confine two people in cells designed to hold only one inmate, in what is believed to be violation of Border Force regulations.

"They are being stuck together in cells for up to 23 hours a day. One person has to sleep on the floor. There's nothing to do in there – there's no way to shower, they can't contact their families,"  a source said.

Others have been forced to sleep in common areas.

"If Serco and Border Force keep pushing, pushing, pushing, this place is going to go off. Conditions are worse than at a max-security prison on the mainland."

The government is believed to have spent millions of dollars hiring special charter flights to transport so called "501 detainees" to the small island in the Indian Ocean.

Some inmates are facing indefinite detention as it is considered unlikely their countries of origin will accept their repatriation due to their criminal convictions. Others have been sent to Christmas Island before they could face criminal trials.

cvedelago@theage.com.au