​The first call between world leaders should be an easy, low-key affair - some pleasantries, an acknowledgment of the importance of the relationship, possibly a tentative suggestion of a state visit.

But the weekend chat between US President Donald Trump and Prime Minister Malcolm Turnbull was anything but ordinary. And people are now wondering how the extraordinary details were made public.

There were only a handful of witnesses to the conversation which took place early on Saturday afternoon, Washington time.

Mr Trump's controversial chief strategist Stephen Bannon, national security adviser Michael Flynn and White House press secretary Sean Spicer were in the Oval Office when the call was made. Mr Turnbull was the fifth world leader Mr Trump had spoken to that day. It was just after 5pm and the newly minted president had earlier finished calls with the leaders of Japan, Germany, Russia and France.