McCain, who is one of Trump's most vocal Republican critics, told the crowd in Sydney that he understands why Australians and other U.S. allies would doubt America's commitment to them after Trump took office.
"I realize that I come to Australia at a time when many are questioning whether America is still committed to these values," McCain, who was the 2008 Republican presidential nominee, said. "And you are not alone. Other American allies have similar doubts these days. And that is understandable."
"I realize that some of President Trump's actions and statements have unsettled America's friends," McCain added. "They have unsettled many Americans as well."
Trump had just met with other leaders at the G7 summit in Italy last week. Following the gathering, German Chancellor Angela Merkel slammed the president, saying the European Union can no longer "fully count on others" in an apparent jab at the U.S.
Trump's relationship with Australia came under fire during the first days of his presidency in January, when reports of an uncomfortable phone call between Trump and Australian Prime Minister Malcolm Turnbull made waves. The Washington Post reported that Trump hung up the call early after criticizing a refugee deal between the two countries.
The two leaders met for the first time in person in May in New York City, where they said they have a strong relationship.
Amid the tension, McCain told the Australian crowd that there was a "real debate" in America over what role the country should play in the world.
"And frankly, I do not know how this debate will play out," McCain said. "What I do believe, and I do not think I am exaggerating here, is that the future of the world will turn, to a large extent, on how this debate in America is resolved."