SYDNEY, Australia — Five days after 50 Muslims in New Zealand were killed in an attack attributed to an Australian white supremacist, Australia’s prime minister, Scott Morrison, unveiled a plan he said would address a fundamental challenge to the nation.

But it was not a proposal to combat hate groups and Islamophobia. It was a cut to immigration.

The government’s plan, which had been in the works for months, is a potential turning point for a nation that has been shaped by newcomers since its days as a British penal colony and that has presented itself in recent years as a model of how immigration, properly managed, can strengthen a country.

Now, amid a global backlash against immigration that has upended politics in the United States, Britain and much of Europe, even Australia is reversing course, turning away from a policy of welcoming skilled foreigners that helped fuel decades of economic growth — and transformed a nation once closed to nonwhite immigrants into a multicultural society.

Mr. Morrison presented the move as a reaction to crowding in the nation’s largest cities, which has led to congested commutes and costlier housing. “This plan is about protecting the quality of life of Australians right across our country,” he said.