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US President Donald Trump’s conversation with Prime Minister Scott Morrison, seeking Australian co-operation with a US Justice Department ­investigation of the Mueller ­inquiry, is "procedurally routine, yet politically extraordinary", according to United States Studies Centre CEO Simon Jackman.

The fallout over the September phone call between the two leaders, as well as Attorney-General William Barr's investigation into the origins of the FBI investigation into links between Russia and the Trump campaign, has seen the impeachment and whistleblower scandals spread beyond Ukraine and the United States, to Australia, Italy and the United Kingdom.

“[The call was] not about contributing with defence for an operation we might be doing together, or for a trade matter,’’ Professor Jackman told The New York Times. ‘‘This was: ‘What were the circumstances under which Australia felt compelled to pass on this intelligence about election interference that helped me become president?’ We’re in a whole new category.”