Intelligence agency ASIO has made startling allegations - of plots to kill, training of militants, forged documents and people smuggling - to justify the indefinite incarceration of dozens of refugees as security threats to Australia.

But ASIO has given no detailed explanations to back up its assertions, raising doubts about a new review into the cases and a possible escape from what has been dubbed a ''legal black hole'' of indefinite detention.

A carefully worded summary of secret ASIO assessments has finally been sent to most of the 55 refugees branded a risk to national security and held in immigration detention without charge - some for up to four years.

Fairfax Media has compared several of the letters outlining the ASIO claims - but stripped of any secret intelligence - and agreed not to name the individuals held in detention.

Most are Tamils, including women and their children, who fled the Sri Lankan government's bloody civil war with the separatist Tamil Tigers.

All of the 55 are said to be ''likely to engage in acts prejudicial to Australia's security'' but for reasons that are varied or vague.

In one case, ASIO makes a broad accusation against a man of engaging in ''people smuggling activities'' and helping Tamil Tiger fighters obtain false identities.

''ASIO assesses he is associated with individuals involved in organising and facilitating irregular maritime ventures to Australia,'' the letter says.

It goes on to claim, ''he is likely to continue involvement in people smuggling activities in Australia should he be granted a permanent visa''.

The man has not been charged with criminal offences and, despite ASIO alleging he gave misleading information in his refugee claim, he was formally recognised to have a well-founded fear of persecution.

The Australian government, a signatory to the refugee convention, cannot return the man to Sri Lanka, but other countries will not accept him with a negative security assessment.

Immigration Minister Brendan O'Connor has refused to grant a visa to any of the refugees with adverse assessments from ASIO, even to live under strict control orders, preferring to leave them in indefinite detention.

Jane Dixon, of civil liberties organisation Liberty Victoria, said the aftermath of the Boston Marathon attacks made it a difficult time to get the community to empathise with the 55 refugees. But she said there was ample scope under existing law to either charge the group or release them on conditions similar to bail.

''If [the authorities] have reliable evidence of connection to a terrorist organisation, why wouldn't they be charging them and putting them up before the courts?'' Ms Dixon said.

The Tamil Tigers were never classified as a terrorist organisation in Australia, although reports in 2007 indicated the Howard government considered such a ban.

Australian citizens or permanent residents can challenge an ASIO finding in the Administrative Appeals Tribunal - with the agency permitted to keep its reasons secret - but the refugees have no right of appeal.

A group of 27 men with adverse assessments who are locked in a Broadmeadows detention centre ended a 10-day hunger strike last week that was intended to draw attention to their plight.

ASIO had resisted providing any information to the group of 55, fearing to do so would compromise sensitive intelligence sources.

But a review system introduced after a High Court challenge last year has allowed the refugees to request that former Federal Court judge Margaret Stone examine their case.

All 55 have since applied for review and in recent weeks been granted the first insight into the suspicions against them.

ASIO has given 53 summaries of its reasons, with the remaining outlines expected by the end of this month.

Justice Stone heard the first oral evidence on Monday from two of the refugees.

But although the refugees will be told the outcome of the review, ASIO cannot be compelled to change its assessment.

The refugees and their supporters fear unless the impasse is broken the group may never be released.

The Coalition, tipped to win the federal election, has flagged it does not believe any review of the ASIO assessment is needed.

Shadow attorney-general George Brandis told ABC radio the group had ''no one to blame but themselves''.

Sydney University international law specialist Ben Saul - who is pursing a case against indefinite detention with the United Nations - said not to outline detail of the allegations against a person made it impossible to effectively challenge a case.

''You can have an argument about just how much disclosure is required to give someone a fair hearing … but to just make a bald assertion someone is a threat to national security goes nowhere near meeting that standard,'' Professor Saul said.

An ASIO spokesman said the summary of reasons for adverse security assessments was ''limited to information that can be provided to the eligible person without prejudicing the interests of security'' but that Justice Stone had full access to the information - including classified material.

''Disclosing classified national security information to the subject of a security assessment would jeopardise ASIO's ability to provide advice to protect Australia, its people and interests,'' the spokesman said.

ASIO director-general David Irvine told a parliamentary hearing last month that it was not the intelligence agency's job to make a judgment whether a person might be a criminal.

''That is for someone else to do,'' he said, adding ASIO was charged with assessing whether someone was likely to be a threat to national security, which could include espionage, sabotage or politically motivated violence.

ASIO is not known to have made any adverse assessments of refugees since last October, when the review system was introduced.

A spokeswoman for Justice Stone said it was not possible to give a time frame for completing a review of a case.

''The time frame for individual cases depends on a number of factors, including the amount of material to be reviewed, the timeliness with which written submissions are received and oral submissions can be scheduled, and other factors beyond our control,'' she said.

ASIO Links Refugees To Kill Plots, Militant Training