Washington DC | Australia's ambassador Joe Hockey has hit back at one of Donald Trump's closest allies, Lindsay Graham, after the Senator issued a thinly-veiled accusation that Alexander Downer spied against the president's 2016 campaign.

Republican Senator Graham, who has morphed over the years from fierce Trump critic to devoted supporter, wrote to Mr Hockey late on Wednesday (Thursday AEST) demanding that Australia assist Attorney-General William Barr's investigation into the so-called Papadopolous affair.