A total of 15 boats containing 429 asylum seekers have been turned back since the Australian government enacted its Operation Sovereign Borders (OSB) policy, the immigration minister, Peter Dutton, has said.

Some turnbacks to Indonesia were undertaken without the support of local authorities.

“We work with Indonesia as closely as we can, and in some circumstances, we see activities which we work with [the support of Indonesian authorities],” the operation’s commander, Lieutenant General Angus Campbell told reporters on Wednesday. “In other cases we undertake turnback operations without.”

Campbell said the boats were returned only to Indonesia and Sri Lanka.

“There are turnbacks, there are also those activities where we work with another country to return those in what you could describe as a take back. And there are some circumstances where we might assist persons in a safety of life at sea circumstance,” Campbell said.

“Only one vessel has arrived in Australia in 2014, and all of those aboard that vessel were transferred to Nauru,” Campbell said. “There were no known deaths as a result of our OSB activities in 2014.”

Dutton said the Coalition had succeeded in its election promise of stopping the boats.

“OSB and our turnback policy has restored the integrity of our borders,” Dutton said.

Then immigration minister Scott Morrison introduced the OSB policy in December 2013.

Dutton’s acknowledgement of the turnbacks is a move away from his predecessor’s policy not to discuss so-called “on the water matters”.

He refuted claims that Australia was sending asylum seekers back home to potential harm, saying Australia was still bound by international law principles.

Dutton said there were still seven children in mainland immigration detention awaiting security clearances before they could be released into the community.

Six adult refugees have been moved from the Manus Island detention centre to the refugee transit centre in nearby Lorengau, where they will be housed temporarily before being moved to another part of Papua New Guinea.

But the PNG prime minister, Peter O’Neill, has told the ABC that he believes the majority of the more than 1,000 asylum seekers in the Manus detention centre are not genuine refugees but economic opportunists who do not have legitimate claims for protection.

“I think many of them are just out there trying to have economic opportunities that Australia and other countries offer to them and that is why they’re seeking refugee status,” O’Neill said.

“I think it will be in the very small numbers [found to be legitimate refugees]. Most of the other people who are in the processing centre: we’re now talking to their governments and we will start repatriating many of them in a very short time.”

He said his government would quickly return people found not to be refugees to their countries of origin.

“We are now trying to work with some of their governments where they come from, like Iraq and Iran … we are trying to determine this as quickly as possible

Iran does not accept involuntary returns of its citizens, and Iraq, historically, has also refused to accept citizens repatriated against their will.

The United Nations has previously raised concerns about PNG’s refugee assessments, arguing there are “shortcomings in the legal framework”, no rights for review, and concerns about the efficiency and fairness of the process.

Under international law – codified in the refugees convention, to which Australia and PNG are parties – a refugee cannot be returned to “territories where his/her life or freedom would be threatened”.

And Australia retains an international legal responsibility for people moved to offshore detention centres.

If PNG were to return a person forcibly who was later found to be a refugee with a genuine fear of persecution to a place where they faced harm, Australia would be in breach of its international obligations, under “chain refoulement”, where refugees are sent to a safe third country which ultimately returns them to persecution.

In September 2014, the incoming United Nations high commissioner for human rights, Zeid Ra’ad Al-Hussein, said that Australia’s asylum policies were ‘‘leading to a chain of human rights violations, including arbitrary detention and possible torture following return to home countries’’.