Earlier reports suggested 20 refugees were allowed to leave detention centres in Victoria, NSW, and Queensland.
An ASRC spokesperson said it was able to confirm 18 people were released late on Friday night, although the number could be higher.
Half of the refugees were released from Melbourne's Park Hotel, the spokesperson said, while the others were from the Brisbane Immigration Transit Authority and Sydney's Villawood Immigration Detention Centre.
The Department of Home Affairs has declined to confirm the reports, with a spokesperson saying it would not comment on individual cases.
The ASRC says there are more than 30 refugees still in Australian detention centres.
Nearly 230 refugees have been released from onshore detention centres since December 2020.
ASRC advocacy director Jana Favero said the government has released the refugees from detention in a "shambolic and secretive" manner.
"After nine years of being held against their will for no reason, the people released deserve so much better," she said in a statement.
"The mental and physical health impacts of being held in a detention centre are enduring and require permanent protection and access to mainstream social support."
It comes three weeks after 13 refugees were released from detention centres in Melbourne and Brisbane.
At the time, advocates said nine men were released from the Park Hotel, along with one other in Broadmeadows and three men in Brisbane.
Park Hotel detainee, Iranian refugee Mehdi Ali, was also resettled in the United States last month, after nine years in detention.
The Park Hotel gained national attention in January when tennis champion Novak Djokovic was briefly detained there.
Djokovic was ultimately deported on the eve of the Australian Open after the Federal Court upheld the immigration minister's decision to cancel his visa.
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