ATTORNEY-General Philip Ruddock has denied local or foreign political influence played a part in the impending deportation of American peace activist Scott Parkin.

Mr Ruddock said the 36-year-old history teacher had not been accused of a crime and his "political activities were not relevant to the decision".

"The reason he's in custody is because his visa has been cancelled. The reason his visa has been cancelled is because he's received an adverse security assessment," Mr Ruddock said on ABC radio.

"ASIO is responsible for protecting the Australian community from all forms of politically motivated violence, including violent protest activity, and they've made an assessment in relation to those matters."

Mr Ruddock said Mr Parkin had arrived in Australia on a valid visa.

"It's certainly the case that ASIO did not issue an adverse security assessment at the time of his (visa) application," Mr Ruddock said.

"The assessment has to be made upon matters relating to politically motivated violence, including violent protest activity," he said.

Mr Ruddock said ASIO had made an objective decision.

"The Australian community expects to be protected from politically motivated violence."

He denied the matter was aimed at testing the Australian public's reaction to tough anti-terror detention laws.

Mr Parkin's friend and fellow activist Ian Murray said Mr Parkin had never espoused violent political action.

"Scott believes in people power and non-violent direct action," Mr Murray said on ABC radio.

"There's a big difference between people power and non-violence and terrorism and it seems like Mr Ruddock and the agencies under his control can't seem to make that distinction."

He said the Melbourne activism workshop that Mr Parkin was about to facilitate when detained on Saturday was focused on sharing lessons from the US peace movement.

"We were going to run a four-hour workshop where Scott was going to share his experiences working with a group called Houston Global Awareness, which in its mission statement has a commitment to non-violence, to non-violent direct action," he said.

"The implication that the minister makes that Scott could somehow be involved in political violence I find highly offensive."

Federal opposition legal spokeswoman Nicola Roxon said Australians were being asked to trust Mr Ruddock's judgment.

"We are very concerned if people who are peaceful, and if this man is, if they simply are expressing views that are not in line with the Government, that is not a reason to detain and deport people."

She said Mr Parkin would have had more protection if he had been arrested, rather than being detained by the Immigration Department.

According to information on ASIO's website, "politically motivated violence" means "acts or threats of violence or unlawful harm that are intended or likely to achieve a political objective". ASIO says it is not concerned with lawful advocacy, protest or dissent.

"This Act shall not limit the right of persons to engage in lawful advocacy, protest or dissent and the exercise of that right shall not, by itself, be regarded as prejudicial to security," the website says.

The law covers acts that involve violence or are likely to lead to violence that are directed at overthrowing or destroying the Government, offences punishable under the Crimes (Foreign Incursions and Recruitment) Act 1978, the Crimes (Hijacking of Aircraft) Act 1972, the Crimes (Protection of Aircraft) Act 1973 or offences punishable under the Crimes (Internationally Protected Persons) Act 1976.

It also includes acts that threaten or endanger any person or people specified by ministerial direction.

"The intelligence that ASIO collects need only be relevant to a risk that such conduct may be engaged in, or to a reasonable apprehension that it is being or has been engaged in,'' the website says.

Meanwhile, Victorian Premier Steve Bracks said the reasons for Mr Parkin's detention should be made public.

"I think we need an explanation," Mr Bracks said.

"To not have an explanation brings into disrepute the very law which is being applied to put him into jail currently and to suspend his visa arrangements.

"Now, there might be a case there, but publicly they should have explained."

Mr Bracks said the detention of the American activist was a federal matter and the Victorian Government had been given no warning or explanation about the incident.

TONY EASTLEY: An American peace activist could be deported from Australia as early as today because he's considered to be a threat to national security.

The Department of Immigration revoked Scott Parkin's six-month visitor visa over the weekend, sparking concerns about the Federal Government's use of anti-terrorism laws.

Mr Parkin's lawyers say they can only surmise that he's being politically persecuted for participating in anti-war rallies since arriving in Australia.

He's being held in a Melbourne prison, at his own expense, until he's deported.

Tanya Nolan reports.

TANYA NOLAN: When Texan-based peace activist Scott Parkin declined to be interviewed by ASIO last week, he contacted community lawyer Marika Dias.

Ms Dias says three days later, the Department of Immigration revoked Mr Parkin's six-month visitor's visa, on the grounds that he's a risk to national security.

MARIKA DIAS: It's difficult to comprehend, it's a very vague term, it's difficult to understand exactly what the basis for that assessment might be.

It certainly hasn't breached any of the conditions of the visa, he's not broken any laws, he isn't suspected of breaking any laws, so one can only assume that it is the political dissent that he's been involved in.

TANYA NOLAN: That political dissent is a number of anti-war rallies Mr Parkin has taken part in since arriving in Australia at the start of June.

What's sparked concern in Mr Parkin's case, is why immigration officials have only acted now in declaring him a security threat, after issuing him a visa, and after he's already spent more than three months in Australia on holiday.

The Department of Immigration will only say that it's conducted a security assessment on Mr Parkin, and concluded that he doesn't meet the character requirements under the Migration Act.

Greens Senator Bob Brown has been trying to get more answers from the Immigration Minister Amanda Vanstone, but in the meantime has come to some conclusions of his own.

BOB BROWN: There is either here a failure of the security clearance which led to his visa, or there is a political move to get him from out of the country because he's embarrassed the Bush administration.

TANYA NOLAN: Because the Immigration Department takes advice from agencies such as ASIO on issues of national security, the Attorney-General is also fielding questions about the exact threat Mr Parkin poses to Australia.

But Attorney-General Philip Ruddock isn't elaborating either.

PHILIIP RUDDOCK: I understand that the decision was based upon a security assessment, and you know security assessments are not matters about which I can comment in any detail.

TANYA NOLAN: A specialist in immigration law who's been asked to assess Mr Parkin's case, Julian Burnside QC, says he's never heard of the security assessment provision of the Migration Act being used retrospectively.

But he says recent counter-terrorism laws give the Federal Government great powers in determining what constitutes a threat to national security, and he says this case highlights concerns about how these laws are being applied.

JULIAN BURNSIDE: They may be right or they may be wrong, but it looks very mysterious that all of a sudden, after weeks, they've decided that he's a national security threat.

TANYA NOLAN: But don't they have that right?

JULIAN BURNSIDE: They've got the right, but the question is whether that right has been exercised in a way that provides sufficient protection for ordinary citizens and visitors to Australia.

TONY EASTLEY: Lawyer Julian Burnside QC, ending Tanya Nolan's report.