Prime Minister Malcolm Turnbull says offshore processing of asylum seekers is harsh but any relaxation would put people smugglers straight back into business.
That would lead to more deaths at sea, as occurred when the former Labor government eased the Howard government's tough stance on boat arrivals.
"Any weakness on our part will be exploited by them and the consequence will be women and children and families drowning at sea," he said on the ABC's Q and A program.
"So it's a tough choice but that's my job to make tough choices to defend Australia, to defend the integrity of our borders."
The prime minister faced some tough questioning from the audience on the issue, with one by video from a Kurdish-Iranian asylum seeker on Manus Island who asked: "Why am I still in this prison after three years?"
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Mr Turnbull said he was sure he would rather come to Australia but that option wasn't open to him.
"None of us have hearts of stone. All of us understand how harsh it is, our policy is, in terms of its impact on particular individuals," he said.
But allowing all those on Manus Island to settle in Australia would be the biggest marketing opportunity for the people smugglers who were itching to get back into business.
"The boats would be setting off again. It would be starting up," he said.
Mr Turnbull said the alternative to the government's tough stance wasn't a theoretical consideration.
When Labor eased the Howard government policy, 50,000 people arrived by boat, with 1200 and likely many more, drowning at sea, he said.
"Now, I grant you it is tough. It is tough. But the alternative is far worse," he said.
