The Australian Border Force last confirmed to AAP that 18 detainees had tested positive on January 13 but is yet to respond to requests for further updates sent since then.
On Tuesday, SBS News reported ABF had confirmed that number had risen to 33, 21 of whom had recovered.
One detainee told AAP their roommate had tested positive and been moved to isolation on Wednesday.
Their roommate's positive case came a day after the detainee received a response to a two-week-old request to be moved to a single room for social distancing purposes.
The response said maintaining social distancing wasn't required for detainees who shared rooms if neither had tested positive or had symptoms.
The detainee says they're now scared to get tested, fearing a positive result that move them into isolation too.
Refugee Action Coalition spokesperson Ian Rintoul told AAP he has consistent contact with detainees inside and believes the 33 cases reported so far is an "underestimate" of the "disaster" outbreak.
"I know there's at least 39 people and the possibility that there are more," he says.
The outbreak was also driving noticeable absenteeism among staff, Mr Rintoul said.
Serco, a contracted operator of the detention centre, directed AAP requests for comment to the Australian Border Force, as did the office of Immigration Minister Alex Hawke.
