Police and immigration officials are searching for 11 young asylum seekers who are thought to have fled from their homes after two others were taken to a detention centre last week.

Last Thursday two Vietnamese asylum seekers were taken from their high school in Adelaide's north by immigration officials.

The teenagers, who had been living in residential housing for some time, were sent to a detention centre believed to be either in Perth or Darwin.

The young detainees had their community detention revoked by the Immigration Department but the Government is yet to reveal its reasoning behind the decision.

Ian Rintoul from the Refugee Action Coalition says the incident resulted in a group of 11 asylum seekers aged 14-18, who were also living in the community, fleeing their homes.

"The Department seems to revel in actions that demonstrate their complete control over people who should be in their care," Mr Rintoul said.

"The Department has put the fear of God into these people - the fear of actually being returned is that something very nasty is going to happen - for no good reason."

Greens 'appalled' by Government's handing of the runaways

Greens Senator Sarah Hanson-Young found out on Friday the children had gone missing.

The initial number was seven, but that was revised up today.

She says it is outrageous that almost a week on, they have not been found.

"These are young kids. They're frightened, they've come to Australia asking for help, they've been through immigration detention already, they are obviously afraid of being sent back there," she said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 48 seconds 3 m 48 s Listen to Caroline Winter's report Download 7 MB

"And if this was any other group of children, there'd be a massive public appeal to try and find them."

Senator Hanson-Young has written to Immigration Minister Scott Morrison about the issue.

She says she is appalled by how it has been handled, and that she fears for the children's safety.

"The Minister is the legal guardian of these children, yet he's also the person who decides whether they are locked up, and for how long," she said.

"It's very hard to have the person who is legally responsible for their welfare also being the person who frightens the hell out of them with the threat of detention."

Opposition calls for more transparency in immigration matters

Senator Hanson-Young believes the two young detainees were removed because they did not have the correct identity documentation.

But the Immigration Department has not confirmed that the pair had been taken or any details about any missing youths.

Shadow Immigration Minister Richard Marles says there should be more transparency in affairs concerning asylum seekers.

"This is a portfolio which needs to be run with a much better degree of clarity and information in the public domain, and information where it's appropriate as well," he said.

"The culture of secrecy that pervades throughout the way in which Scott Morrison does his work, is having an impact at every level, and this is a very sad example of it."

Refugee advocates are calling on the Immigration Minister to be more transparent about the details of the case, which they say will be causing unnecessary fear among the group who are on the run.

"It's very clear that people live in constant anxiety," Mr Rintoul said.

"They've got no confidence about the future, there's no surety about the future, they very often do not understand what the processes that they're going through, what they're subject to, when they've been at the whim of immigration decisions."

In a statement, South Australian police have said they are working with the Immigration Department and Border Protection on the issue, and that there are no concerns for the safety or welfare of those missing.

A request for an interview with the Immigration Minister Scott Morrison, and questions put to his office, were not responded to before PM went to air.