Asked about the call after delivering the annual Lowy Lecture in Sydney on Thursday night, the prime minister said it was "unremarkable".
"It wouldn't matter which president or which attorney general ... was conducting an official investigation into whatever matter. It would be extraordinary of any prime minister in those circumstances to deny what was a very straightforward request."
He noted "we're not the subject of the investigation" and "it's a matter for US domestic politics".
Mr Morrison has faced criticism around the call throughout the week.
Labor Leader Anthony Albanese accused Mr Morrison of prioritising boosting Mr Trump's re-election chances rather than advancing Australia's interests.
Dangers of 'negative globalism'
The Sydney speech followed Mr Morrison's trip last month to Washington DC, where he became the first Australian prime minister since John Howard in 2006 to be invited to a White House state dinner.
On Thursday, Mr Morrison talked up both his relationship with Mr Trump and Australia's relationship with the US more broadly.
"Our alliance with the United States is our past, our present and our future. It is the bedrock of our security," he said.
He also rejected the "the binary narrative" that Australia needs to choose either the US or China.
"China is a global power making significant investments in military capability as a result of its extraordinary economic success," he said.
However, while his speech was broadly optimistic, he warned against what he called "negative globalism".
"We should avoid any reflex towards a negative globalism that coercively seeks to impose a mandate from an often ill-defined borderless global community," he said.
"And worse still, an unaccountable internationalist bureaucracy. Globalism must facilitate, align and engage, rather than direct and centralise. As such an approach can corrode support for joint international action.
"Only a national government, especially one accountable through the ballot box and the rule of law, can define its national interests. We can never answer to a higher authority than the people of Australia."
The prime minister told attendees the Australian government would do more to lead the conversation on issues related to its key national interests, which he identified as global stability, open international trade and aid transparency.
He said he had requested DFAT audit the international bodies and laws most relevant to Australia's welfare so the government could do more to shape them.
The prime minister also dismissed the idea that Australia's response to climate change is damaging our relationships with Pacific countries, calling the region a "family".
The Lowy Lecture address is the flagship event of the Lowy Institute foreign affairs think tank.
Past Lowy Lectures have been delivered by German Chancellor Angela Merkel, former UK Foreign Secretary Boris Johnson, former CIA Director David Petraeus and News Corp Chairman Rupert Murdoch.
Additional reporting from AAP