He declined to specify what those classified measures were, other than to announce the reopening of the camp on Christmas Island, a remote Australian territory some 2,300 kilometres (1,400 miles) northwest of the western city of Perth.
"We are disappointed by the announcement today that Christmas Island is to reopen".
Home Affairs Minister Peter Dutton also said people smugglers were already using the law changes in their marketing.
"If they don't come, it will be because of the work and the decisions we are now taking and the actions we are putting in place", Morrison said.
Labor workplace relations spokesman Brendan O'Connor said the Prime Minister had chosen to "misrepresent the truth" and "lie to the Australian people" in an attempt to gain political advantage when he should be safeguarding the national interest.
Morrison added: "My job now is to ensure that boats don't come, my job now is to do everything in my power to ensure that what the parliament has done to weaken our borders does not result in boats coming to Australia".
The Labor leader said the Coalition should be "ashamed of themselves for luring people to Australia by somehow implying that this Government hasn't got strong borders".
The bill was first mooted by independent MP Kerryn Phelps past year after she won the Liberal Party stronghold of Wentworth when former prime minister Malcolm Turnbull was ousted in a party coup.
As critics of the prime minister's hard-line immigration policies were still celebrating their success, Morrison announced that the Christmas Island camp would have to be reopened instead, "both to deal with the prospect of arrivals as well as dealing with the prospect of transfers".
Australia has come under fire by the United Nations and human rights organizations for allowing asylum-seekers to languish for years on the Pacific island under inhumane conditions.
Last year, Australians were horrified by reports of a mental health crisis among children on Nauru - including cases of attempted suicide.
Labor says Scott Morrison is talking down Australia's border protection regime by deliberately misrepresenting the new medivac laws, which will make it easier for refugees on Manus and Nauru to be transferred to Australia for medical treatment.
Morrison last week ruled out calling a snap election if his government was defeated over the "stupid" bill.
His Liberal party is keen to deliver a surplus budget before the election to underpin its claim to be the best custodians of Australia's finances.
Labor made a strategic move to avoid turning the bill into a test of confidence in the government, withdrawing part of the medical transfer scheme that required funding to pay for medical experts to review transfers. The center, located on the Indian Ocean, held thousands of migrants and had closed down in 2018.
The last 35 detainees were taken off the island last October, when the camp was shuttered.