(CNN) The government of Prime Minister Tony Abbott has revealed Australia has turned back 633 asylum seekers on 20 boats as part of a strict but highly contentious migration policy.

Minister for Immigration and Border Protection Peter Dutton revealed the numbers in a statement on Thursday , which marked the one-year anniversary of the last boat to arrive on Australian shores. Canberra usually declines to provide specific details on boat turn-backs.

"We have had 20 successful turn-back operations," he told reporters. "We have had 633 people that would have arrived otherwise on those ventures."

Controversial policy

Australia started turning boats backs in December of 2013, under the controversial Operation Sovereign Borders, shortly after Abbott was elected to power.

Abbott responded to the U.N. reports at the time by saying they were willing to do whatever it takes to stop the boats whether it was "by hook or by crook."

Dutton added that the last turn-back concerned a boat with 46 people that was returned to Vietnam, but declined to break down how many boats had arrived this year.

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'More boats would have arrived'

"I think the important point to remember in all of this is had those 20 boats arrived, 200 boats or 2,000 boats would have followed them," Dutton said.

"If people smugglers see ventures getting to Australia successfully -- even one venture -- that is a green light for the people smugglers to be back in business."

However, the Refugee Council of Australia (RCOA) criticized the announcement saying the government should properly review claims for asylum rather than turn migrants back at sea, and allow monitoring from the U.N. refugee agency or other independent body.

"This is not the behavior of a government that ensures that it lives up to its obligation to protect persecuted people; this is the behavior of a government that has something to hide," RCOA chief executive officer Paul Power said.

"Refugee claims are often very complex and it is simply not possible to assess these claims fairly or accurately through a rushed screening process on the high seas."