The two women and one man are believed to be Fijians, and began their protest from early morning.

Lawyer George Newhouse, who acted on behalf of the family of 36-year-old Fijian national Josefa Rauluni, who leapt to his death in 2010, was visiting clients on Monday and witnessed the protest.

He said police did not appear to be on site and that centre management spreading mattresses below the trio would do little to prevent them from jumping from the roof.

''People jump over mattresses if they want to kill themselves … there's no police there at the moment. But when police are on a roof in this sort of heat, it is a high-risk situation …''

A Department of Immigration and Citizenship spokesman confirmed three ''clients'' were engaged in a rooftop protest and trained negotiators were on site.

''The detention services provider is engaging with them and encouraging them to come down,'' he said.

In December, New South Wales Coroner Mary Jerram found neither detention centre operator Serco, nor the department, had fulfilled its obligations to Mr Rauluni, or Iraqi asylum seeker Ahmed al-Akabi and Briton David Saunders, who both hanged themselves in the shower in 2010.

''When a government chooses to maintain a detention system it carries a heavy responsibility,'' she said.

She recommended detention operators use trained police negotiators to advise on high-risk situations.

The department spokesman said involving police was an ''operational'' decision.

Refugee Action Coalition spokesman Ian Rintoul said the protest was sparked when a female Fijian asylum seeker, scheduled for deportation on Monday, climbed onto the roof with two others at 7am to protest against her deportation.

He said the woman's husband was scheduled for removal today.

"The Immigration Department should withdraw the removal notice for both the wife and husband,'' Mr Rintoul said.

''That is the first step to de-escalate the situation and remove the threat of forcible removal.''