In an extraordinary address in Sydney on Tuesday evening, Republican giant John McCain begged an Australian audience that included two former prime ministers, diplomats and business leaders for patience and even leadership to sustain the alliance between the US and Australia during the Trump presidency.

Speaking as a guest of the United States Studies Centre in the State Library's grand reading room, McCain began his speech with a reverent reflection upon the shared sacrifices of Australian and American soldiers over a century, before acknowledging the uncertainty the new administration had caused around the world.

"I realise that I come to Australia at a time when many are questioning whether America is still committed to these values," the US senator said early in his 30-minute address. "And you are not alone. Other American allies have similar doubts these days. And this is understandable.

"I realise that some of President Trump's actions and statements have unsettled America's friends. They have unsettled many Americans as well. There is a real debate under way now in my country about what kind of role America should play in the world. And frankly, I do not know how this debate will play out.