Cabinet documents obtained by the ABC show former immigration minister Scott Morrison requested ASIO in 2013 to delay security checks so asylum seekers would miss a deadline to obtain permanent protection visas.
It is unclear whether ASIO complied with the written request.
But Mr Morrison said in a statement on Tuesday, "As minister for Immigration and Border Protection, it was my policy and practice to put Australia's national security interests first."
The prime minister backed up his cabinet colleague, saying Mr Morrison had "stopped the boats".
"We make no apologies for sending the clearest message to the people smugglers and to their would-be customers: 'If you think you can come to Australia on a people smuggler's boat, you're wrong. You won't. You won't get here, you will not become a permanent resident'," Mr Turnbull told reporters in Sydney.
"Keeping our borders secure is a critical obligation and responsibility of government."
Mr Morrison's cabinet colleague Greg Hunt would not comment the detail of the leaked documents but defended the government's "strong and clear steps" to save asylum seekers from being lost at sea.
"Everybody in Australia knew that the government was absolutely determined to stop the carnage at sea," he told ABC radio.
"There was a catastrophic human tragedy which was unspeakable and it was unthinkable that that could continue."
Mr Hunt said people who continued to "airbrush" the activities of people smugglers at the time should "take a long, hard look at themselves".
