Immigration Minister Peter Dutton says the Vietnamese government cooperated with Australian authorities in .
Mr Dutton to help prevent more asylum seekers from drowning at sea on their way to Australia.
"We've had only one successful boat arrival, or people-smuggling-venture arrival, since December of last year," Mr Dutton said.
"Bear in mind that we had 50,000 people arrive on 821 boats during the course of the previous government and we've been able to turn back boats where it's been safe to do so," he added.
The Department of Immigration said the most recent figures through March showed 120 Vietnamese were in Australian immigration detention centres.
The department said they made up six per cent of all detainees.
This time last year, there were 674 Vietnamese, making up 15 per cent of the total number of detainees.
The group the Vietnamese Community in Australia said it believed asylum seekers were fleeing Vietnam based on well-founded grounds of persecution by the government.
The group said government authorities were increasingly confiscating land in Vietnam used for worship by the country's Christian and Buddhist communities.
And it said bloggers and human-rights activists were frequently arrested by government officials who saw them as a threat.
The president of the Victorian chapter of the Vietnamese Community in Australia, Bon Nguyen said he feared for the welfare of asylum seekers sent back.
"I would not be surprised if they would actually be punished in a way", Mr Nguyen said.
"For example, they can actually give a direct order to those people around them not to have any connection with them, so that their families then no longer have a job", he added.
"And in Vietnam, if you don't have a job -- or anywhere, if you don't have a job -- how can you support your family?"
Mr Nguyen also questioned the ties being forged between the Australian and Vietnamese immigration authorities in apprehending asylum-seeker boats.
"To push them back to the government that they have run away from, I think that's inhumane", he said.
"As an Australian, I urge all Australian people to actually voice their concern, because this is not portraying the values the Australian people are having."
Refugee advocate Misha Coleman worked at the Australian embassy in Vietnam between 2003 and 2007.
Now executive officer of the Australian Churches Refugee Taskforce, she has heard widespread concern about persecution faced by Christian activists in Vietnam.
Ms Coleman said government authorities in Vietnam harass many Hmong people, decades after they supported the United States during the Vietnam War.
And she said people the government deemed socially unacceptable, including injecting drug users and sex workers, were detained in forced-labour camps.
"Basically, most people live under some sort of fear, threat of retribution or harassment if they're not toeing the line", Ms Coleman said.
"And toeing the line is becoming an increasingly more difficult road to walk in Vietnam", she added.
Ms Coleman said the Australian government was ignoring its international legal obligations by turning asylum seekers back to countries they are fleeing from.
She has called on Australia's ambassador to Vietnam to monitor the welfare of any asylum seekers sent back to Vietnam under the boat-turnback policy.
"To find out whether their claims have been assessed by the Australian government and to give the Australian taxpayer some information about what is going to happen to those people now that they're back in Vietnam", Ms Coleman said.
