A refugee widow and her six-year-old son is expected to taste freedom in Australia on Thursday after ASIO dramatically reversed an assessment that she posed a threat to national security.

The decision followed an internal ASIO review into her case and is not related to an inquiry by a former federal court judge into 55 refugees trapped in indefinite detention inside Australia's immigration network.

Ragavan, 6, who has been held in immigration detention with his mother, Manokala, will be released after an adverse assessment by ASIO was reversed.

It is believed to be only the second time ASIO has rescinded an adverse security assessment of a refugee, raising the hopes of others in the group that their assessments will be reversed. Some have been held for up to four years.

Manokalo fled Sri Lanka's bloody civil war in 2009 with son Ragavan after her husband was killed in a bombing raid.