Opponents accuse him of spreading fear before an impending election.
The bill facilitating medical evacuations of refugees on Manus and Nauru to Australia was approved on Tuesday in the Lower House and ratified by the Senate.
Following that, the Prime Minister on Wednesday said that the government would re-open the Christmas Island centre "to deal with the prospect of arrivals... and transfers" - arguing both were now more likely, the BBC reported.
Morrison said the reopening of Christmas island centre would strengthen the capacity of Operation Sovereign Borders, the government's controversial border protection operation aimed at stopping boat arrivals to the country.
"My job now is to ensure that the boats don't come. My job now is to do everything within my power and in the power of the government to ensure that what the parliament has done to weaken our borders does not result in boats coming to Australia."
Christmas island, an Australian territory located on 380 km from Indonesia's Java island, was until 2012 the point of arrival of boats of undocumented migrants trying to seek asylum in the Oceanic country.
In 2010, a boat of 90 asylum seekers - mostly from Iraq and Iran - sank off the island, killing 50 people on board.
Canberra then toughened its policy against illegal immigration and in 2012 reopened its asylum processing centres on Nauru and Manus island, keeping asylum seekers out of Australia, and where a thousand remain in conditions widely criticized by medical professionals, rights groups and the UN.
Australia has long defended its offshore detention policy by arguing that it stops deaths at sea and disrupts the trade of people smuggling.
The reopening of the centre on Christmas island, which was closed in 2018, has been announced a few months before an election in Australia. The date has not been announced but it is expected to be in May.
Many of the immigrants that Australia intercepts have fled conflict-ridden countries or regions such as Afghanistan, Darfur, Pakistan, Somalia and Syria, or have escaped conditions of discrimination or statelessness such as the minority Rohingyas in Myanmar or the Biduns in the Persian Gulf.