A Vietnam court has convicted four men of people-smuggling after they tried to sail to Australia with a boatload of migrants but were turned back under Canberra's hardline immigration policy.

Australia does not accept so-called boat people from any nation and the country has come under fire for holding migrants in isolated outposts in Papua New Guinea and the island of Nauru.

The four Vietnamese sentenced in southern Ba Ria-Vung Tau province on Tuesday were deported from Australia in June after their boat carrying 21 people was spotted in Australian waters.

Two were given three and three and a half years in prison, while another pair were handed 18-month suspended sentences, a court clerk told AFP on Wednesday.

The Vietnamese migrants, who were reportedly seeking higher paying jobs abroad, were also deported but did not face charges.

The ringleader of the group, 27-year-old Nguyen Giao Thong, had previously fled to Australia in 2013 before he was arrested and returned to Vietnam, according to Thanh Nien newspaper.

Seeking to return, Thong organised the trip with three others and set sail on May 18, reaching Australian waters 20 days later.

The four men were sentenced just days after Canberra and Hanoi signed an agreement on the return of Vietnamese nationals deemed to be in Australia illegally.

Canberra says the tough stance is crucial to deter people-smugglers and prevent deaths at sea. Many of those held in detention centres are from war-torn Afghanistan, Sri Lanka and the Middle East.

Since 2015, Australia has returned 113 Vietnamese nationals from three vessels intercepted at sea.

There are more than 230,000 Vietnamese people living in Australia, according to official figures.